Full text of "Processes and design for manufacturing"

See other formats

SherifD.ElWakil

Processes and Design
for Manufacturing

Second

Processes and Design
for Manufacturing

Second Edition

SherifD. ElWakil

University of Massachusetts Dartmouth

WAVELAND

PRESS, INC.

Prospect Heights, Illinois

To the memory ofMamdouh El-Wakil, M.D., Ph.D.

For information about this book, contact:
Waveland Press, Inc.
P.O. Box 400

Prospect Heights, Illinois 60070
(847)634-0081
www.waveland.com

ISBN 1-57766-255-5

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in
any form or by any means without permission in writing from the publisher.

Printed in the United States of America

7 6 5 4 3 2 1

Chapter 1 Overview 1

INTRODUCTION

1.1 Definition of Manufacturing 1

1.2 Relationship Between Manufacturing and Standard of Living 2

1.3 Overview of the Manufacturing Processes 2

1.4 Types of Production 3

1.5 Fundamentals of Manufacturing Accuracy 4

1.6 The Production Turn 6

1.7 Product Life Cycle 7

1.8 Technology Development Cycle 8

1.9 The Design Process 10

1.10 Product Design: The Concept of Design for Manufacturing 14
Review Questions 16

Chapter 2 Concurrent Engineering 17

INTRODUCTION

2.1 Reasons for Adopting Concurrent Engineering 19

2.2 Benefits of Concurrent Engineering 20

2.3 Factors Preventing the Adoption of Concurrent Engineering 2 1

2.4 The Four Pillars of Concurrent Engineering 22

2.5 Forces of Change 24

2.6 A Success Story: Nippondenso 30
Review Questions 32

iii

iv Contents

Chapter 3 Casting and Foundry Work 33

I NTRODUCTION

3.1 Classifications of Casting by Mold Material 34

3.2 Classifications of Casting by Method of Filling the Mold 52

3.3 Classifications of Casting by Metal to be Cast 58

3.4 Foundry Furnaces 63

3.5 Casting Defects and Design Considerations 68

3.6 Cleaning, Testing, and Inspection of Castings 72

3.7 Castability (Fluidity) 75
Review Questions 76
Design Example 78
Design Projects 81

Chapter 4 Joining of Metals 84

I NTRODUCTION

4.1 Riveting 84

4.2 Welding 84

4.3 Surfacing and Hard-Facing 120

4.4 Thermal Cutting of Metals 121

4.5 Brazing and Soldering 123

4.6 Sticking of Metals 128
Review Questions 130
Problems 133
Design Example 133
Design Projects 137

Chapter 5 Metal Forming 139

I NTRODUCTION

5.1 Plastic Deformation 140

5.2 Rolling 145

5.3 Metal Drawing 155

5.4 Extrusion 158

5.5 Forging 176

5.6 Cold Forming Processes 201
Review Questions 204
Problems 207

Design Example 207
Design Projects 209

Contents

Chapter 6 Sheet Metal Working 211

INTRODUCTION

6.1 Press Working Operations 212

6.2 High-Energy-Rate Forming (HERF) 238

6.3 Spinning of Sheet Metal 241
Review Questions 242
Problems 244

Design Example 245
Design Projects 246

Chapter 7 Powder Metallurgy 248

INTRODUCTION

7.1 Metal Powders 249

7.2 Powder Metallurgy: The Basic Process 254

7.3 Operational Flowchart 258

7.4 Alternative Consolidation Techniques 258

7.5 Secondary Consolidation Operations 263

7.6 Finishing Operations 264

7.7 Porosity in Powder Metallurgy Parts 266

7.8 Design Considerations for Powder Metallurgy Parts 268

7.9 Advantages and Disadvantages of Powder Metallurgy 270

7.10 Applications of Powder Metallurgy Parts 270
Review Questions 274

Problems 275
Design Project 277

Chapter 8 Plastics 278

I NTRODUCTION

8.1 Classification of Polymers 279

8.2 Properties Characterizing Plastics and Their Effect on Product Design 282

8.3 Polymeric Systems 283

8.4 Processing of Plastics 291

8.5 Fiber-Reinforced Polymeric Composites 303
References 328

Review Questions 328
Design Projects 330

vi Contents

Chapter 9 Physics of Metal Cutting 331

INTRODUCTION

9.1 Cutting Angles 332

9.2 Chip Formation 334

9.3 Cutting Forces 339

9.4 Oblique Versus Orthogonal Cutting 343

9.5 Cutting Tools 348

9.6 Machinability 353

9.7 Cutting Fluids 354

9.8 Chatter Phenomenon 356

9.9 Economics of Metal Cutting 356
Review Questions 358
Problems 359

Design Project 360

Chapter 10 Machining of Metals 361

INTRODUCTION

10.1 Turning Operations 362

10.2 Shaping and Planing Operations 379

10.3 Drilling Operations 382

10.4 Milling Operations 392

10.5 Grinding Operations 400

10.6 Sawing Operations 405

10.7 Broaching Operations 407

10.8 Nontraditional Machining Operations 408
Review Questions 411

Problems 413

Chapter 11 Product Cost Estimation 415

INTRODUCTION

11.1 Costs: Classification and Terminology 416

11.2 Labor Cost Analysis 418

11.3 Material Cost Analysis 421

11.4 Equipment Cost Analysis 423

11.5 Engineering Cost 425

11.6 Overhead Costs 425

11.7 Design to Cost 427

Contents vii

Review Questions 427
Problems 428
Design Project 430

Chapter 12 Design for Assembly 431

INTRODUCTION

12.1 Types and Characteristics of Assembly Methods 432

12.2 Selection of Assembly Method 435

12.3 Product Design for Manual Assembly 436

12.4 Product Design for Automatic Assembly 438

12.5 Product Design for Robotic Assembly 445

12.6 Methods for Evaluating and Improving Product DFA 446
Review Questions 459

Design Project 459

Chapter 13 Environmentally Conscious Design
and Manufacturing 460

INTRODUCTION

13.1 Solid- Waste Sources 462

13.2 Solid-Waste Management 464

13.3 Guidelines for Environmentally Conscious Product Design 469

13.4 Environmentally Conscious Manufacturing 472

13.5 Environmental Protection and Pollution Control Legislation 473
Review Questions 475

Chapter 14 Computer-Aided Manufacturing 476

INTRODUCTION

14.1 Numerical Control (NC) 476

14.2 Computerized Numerical Control (CNC) 494

14.3 Direct Numerical Control (DNC) 498

14.4 Computer-Aided Part Programming 499

14.5 Other Applications of Computer- Aided Manufacturing 514
Review Questions 516

Problems 518

Chapter 14 Appendix 520

viii Contents

Chapter 15 Industrial Robots 523

INTRODUCTION

15.1 Reasons for Using Robots 524

15.2 Methods for Classifying Robots 525

15.3 Components of a Robot 536

15.4 End Effectors 537

15.5 Sensors 540

15.6 Industrial Applications of Robots 541
Review Questions 545

Chapter 16 Automated Manufacturing Systems 547

INTRODUCTION

16.1 Computer-Integrated Manufacturing (CIM) 548

16.2 Group Technology (GT) 556

16.3 Computer-Aided Process Planning (CAPP) 562

16.4 Material-Requirement Planning (MRP) 565

16.5 The Potential of Artificial Intelligence in Manufacturing 566

16.6 Flexible Manufacturing System (FMS) 568
Review Questions 575

Appendix Materials Engineering 577

I NTRODUCTION

A.l Types of Materials 577

A.2 Properties of Materials 580

A.3 Standard Tests for Obtaining Mechanical Properties 580

A.4 Phase Diagrams 590

A.5 Ferrous Alloys 595

A.6 Aluminum Alloys 603

A.7 Copper Alloys 604

References 605

Index 610

At the time the author's first book on processes and design for manufacturing was
published, the main concern of the manufacturing/engineering academic commu-
nity was the erroneous picture of manufacturing as involving little more than manual
training (i.e., manual skills acquired by on-site training in machine shops and the like).
Unfortunately, this distorted view of manufacturing was created and fueled by the
shallow, descriptive, and qualitative manner in which the vast majority of books then
covered the subject. Now, design for manufacturing is a "hot topic," and engineers in
all disciplines are beginning to realize its strategic importance. Many government pro-
grams are aimed at enhancing the efficiency of product development and design. The
present text serves to provide engineering students with the knowledge and skills re-
quired for them to become good product designers.

The design component in this book has been strengthened by adding four new
chapters:

• Chapter 2, Concurrent Engineering

• Chapter 1 1 , Product Cost Estimation

• Chapter 12, Design for Assembly

• Chapter 13, Environmentally Conscious Design and Manufacturing

Also, whenever applicable, chapters have been supplemented by design examples il-
lustrating the interaction between design and manufacturing and showing how prod-
ucts can be designed for producibility, taking factors like the lot size into
consideration. In addition, some design projects, which were previously assigned at
the University of Massachusetts Dartmouth, have been given at the end of several
chapters. Students are encouraged to use computational tools like spreadsheets and
other software for modeling and analysis.

The text has also been supplemented with an appendix that covers the fundamen-
tals of materials engineering. It provides a basis for understanding manufacturing
processes, as well as for selecting materials during the product design process. It is

Preface

aimed at engineering students who have not taken materials science as a prerequisite
for a course on manufacturing processes but is not meant as a substitute for any mate-
rials science textbook.

The author wishes to acknowledge the contributions of the many corporations and
individuals who supplied various figures and photographs or provided software to aid
in producing this book, chief among them Silverscreen. Thanks are also extended to
reviewers of the manuscript:

Mary C. Kocak, Pellissippi State Technical Community College

Zhongming (Wilson) Liang, Purdue University — Fort Wayne

Wen F. Lu, University of Missouri — Rolla

Antonio Minardi, University of Central Florida

Charles Mosier, Clarkson University

Masud Salimian, Morgan State University

Richard D. Sisson. Jr., Worcester Polytechnic Institute

Joel W. Troxler, Montana State University

David C. Zenger, Worcester Polytechnic Institute

Yuming Zhang, University of Kentucky

A note of gratitude also goes to Ana Gonzalez for her hard work in typing the
manuscript. The author wishes to thank Andrea Goldman and Jean Peck for their
encouragement and support. Finally, the author must express his profound gratitude to
his wife and children for their patience as the huge task of completing this second edi-
tion unfolded. God knows the sacrifice they gave.

SherifD. El Wakil
North Dartmouth, Massachusetts

Chapter 1

vervlew

INTRODUCTION

Before learning about various manufacturing processes and the concept of de-
sign for manufacturing, we first must become familiar with some technical terms
that are used frequently during the planning for and operation of industrial man-
ufacturing plants. We also must understand thoroughly the meaning of each of
these terms, as well as their significance to manufacturing engineers. The ex-
planation of the word manufacturing and its impact on the life-style of the peo-
ple of industrialized nations should logically come at the beginning. In fact, this
chapter will cover all these issues and also provide a better understanding of the
design process, as well as the different stages involved in it. Finally, the concept
of design for manufacturing and why it is needed will be explained.

• /^~*\

1.1 DEFINITION OF MANUFACTURING

Manufacturing can be defined as the transformation of raw materials into useful prod-
ucts through the use of the easiest and least expensive methods. It is not enough, there-
fore, to process some raw materials and obtain the desired product. It is, in fact, of major
importance to achieve this goal by employing the easiest, fastest, and most efficient
methods. If less efficient techniques are used, the production cost of the manufactured
part will be high, and the part will not be as competitive as similar parts produced by
other manufacturers. Also, the production time should be as short as possible in order to
capture a larger market share.

The function of a manufacturing engineer is, therefore, to determine and define the
equipment, tools, and processes required to convert the design of the desired product
into reality in an efficient manner. In other words, it is the engineer's task to find out
the most appropriate, optimal combination of machinery, materials, and methods

Overview

needed to achieve economical and trouble-free production. Thus, a manufacturing en-
gineer must have a strong background in materials and up-to-date machinery, as well
as the ability to develop analytical solutions and alternatives for the open-ended prob-
lems experienced in manufacturing. An engineer must also have a sound knowledge of
the theoretical and practical aspects of the various manufacturing methods.

..2 RELATIONSHIP BETWEEN MANUFACTURING
AND STANDARD OF LIVING

The standard of living in any nation is reflected in the products and services avail-
able to its people. In a nation with a high standard of living, a middle-class family
usually owns an automobile, a refrigerator, an electric stove, a dishwasher, a wash-
ing machine, a vacuum cleaner, a stereo, and, of course, a television set. Such a
family also enjoys health care that involves modern equipment and facilities. All
these goods, appliances, and equipment are actually raw materials that have been
converted into manufactured products. Therefore, the more active in manufacturing
raw materials the people of a nation are, the more plentiful those goods and ser-
vices become; as a consequence, the standard of living of the people in that nation
attains a high level. On the other hand, nations that have raw materials but do not
fully exploit their resources by manufacturing those materials are usually poor and
are considered to be underdeveloped. It is, therefore, the know-how and capability
of converting raw materials into useful products, not just the availability of miner-
als or resources within its territorial land, that basically determines the standard of
living of a nation. In fact, many industrial nations, such as Japan and Switzerland,
import most of the raw materials that they manufacture and yet still maintain a high
standard of living.

VERVIEW OF THE
MANUFACTURING PROCESSES

The final desired shape of a manufactured component can be achieved through one or
more of the following four approaches:

1. Changing the shape of the raw stock without adding material to it or taking mater-
ial away from it. Such change in shape is achieved through plastic deformation, and
the manufacturing processes that are based on this approach are referred to as metal
forming processes. These processes include bulk forming processes like rolling,
extrusion, forging, and drawing, as well as sheet metal forming operations like
bending, deep drawing, and embossing. Bulk forming operations are covered in
Chapter 5, and the working of sheet metal is covered in Chapter 6.

2. Obtaining the required shape by adding metal or joining two metallic parts to-
gether, as in welding, brazing, or metal deposition. These operations are covered
in Chapter 4.

1.4 Types of Production 3

3. Molding molten or particulate metal into a cavity that has the same shape as the
final desired product, as in casting and powder metallurgy. These processes are cov-
ered in Chapters 3 and 7, respectively.

4. Removing portions from the stock material to obtain the final desired shape. A cut-
ting tool that is harder than the stock material and possesses certain geometric char-
acteristics is employed in removing the undesired material in the form of chips.
Several chip-making (machining) operations belong to this group. They are exem-
plified by turning, milling, and drilling operations and are covered in Chapter 10.
The physics of the process of chip removal is covered in Chapter 9.

YPES OF PRODUCTION

Modern industries can be classified in different ways. These classifications may be by
process, or by product, or based on production volume and the diversity of products.
Classification by process is exemplified by casting industries, stamping industries, and
the like. Classification by product indicates that industries may belong to the automo-
tive, aerospace, and electronics groups. Classification based on production volume
identifies three distinct types of production: mass, job shop, and moderate. Let us
briefly discuss the features and characteristics of each type. We will also discuss the
subjects in greater depth later in the text.

Mass Production

Mass production is characterized by the high production volume of the same (or very
similar) parts for a prolonged period of time. An annual production volume of less than
50,000 pieces cannot usually be considered as mass production. As you may expect, the
production volume is based on an established or anticipated sales volume and is not di-
rectly affected by the daily or monthly orders. The typical example of mass-produced
goods is automobiles. Because that type attained its modern status in Detroit, it is some-
times referred to as the Detroit type.

Job Shop Production

Job shop production is based on sales orders for a variety of small lots. Each lot may
consist of up to 200 or more similar parts, depending upon the customer's needs. It is
obvious that this type of production is most suitable for subcontractors who produce
varying components to supply various industries. The machines employed must be
flexible to handle frequent variations in the configuration of the ordered components.
Also, the personnel employed must be highly skilled in order to handle a variety of
tasks that differ for the different parts that are manufactured.

Moderate Production

Moderate production is an intermediate phase between the job shop and the mass
production types. The production volume ranges from 10,000 to 20,000 parts, and
the machines employed are flexible and multipurpose. This type of production is

1 Overview

gaining popularity in industry because of an increasing market demand for cus-
tomized products.

1.5 FUNDAMENTALS OF

MANUFACTURING ACCURACY

Modern manufacturing is based on flow-type "mass" assembly of components into
machines, units, or equipment without the need for any fitting operations performed on
those components. That was not the case in the early days of the Industrial Revolution,
when machines or goods were individually made and assembled and there was always
the need for the "fitter" with his or her file to make final adjustments before assembling
the components. The concepts of mass production and interchangeability came into
being in 1798, when the American inventor Eli Whitney (born in Westboro, Massa-
chusetts) contracted with the U.S. government to make 10,000 muskets. Whitney
started by designing a new gun and the machine tools to make it. The components of
each gun were manufactured separately by different workers. Each worker was as-
signed the task of manufacturing a large number of the same component. Meanwhile,
the dimensions of those components were kept within certain limits so that they could
replace each other if necessary and fit their mating counterparts. In other words, each
part would fit any of the guns he made. The final step was merely to assemble the in-
terchangeable parts. By doing so, Eli Whitney established two very important concepts
on which modern mass production is based — namely, interchangeability and fits. Let
us now discuss the different concepts associated with the manufacturing accuracy re-
quired for modern mass production technologies.

Tolerances

A very important fact of the manufacturing science is that it is almost impossible to ob-
tain the desired nominal dimension when processing a workpiece. This is caused by
the inevitable, though very slight, inaccuracies inherent in the machine tool, as well as
by various complicated factors like the elastic deformation and recovery of the work-
piece and/or the fixture, temperature effects during processing, and sometimes the skill
of the operator. Because it is difficult to analyze and completely eliminate the effects
of these factors, it is more feasible to establish a permissible degree of inaccuracy or a
permissible deviation from the nominal dimension that would not affect the proper
functioning of the manufactured part in any detrimental way. According to the Inter-
national Standardization Organization (ISO) system, the nominal dimension is referred
to as the basic size of the part.

The deviations from the basic size to each side (in fact, both can also be on the
same side) determine the high and the low limits, respectively, and the difference be-
tween these two limits of size is called the tolerance. It is an absolute value without a
sign and can also be obtained by adding the absolute values of the deviations. As you
may expect, the magnitude of the tolerance is dependent upon the basic size and is des-

1.5 Fundamentals of Manufacturing Accuracy

FIGURE 1.1

The relationship
between tolerance
and production cost

Tolerance

ignated by an alphanumeric symbol called the grade. There are 1 8 standard grades of
tolerance in the ISO system, and the tolerances can be obtained from the formulas or
the tables published by the ISO.

Smaller tolerances, of course, require the use of high-precision machine tools in man-
ufacturing the parts and, therefore, increase production cost. Figure 1 . 1 indicates the rela-
tionship between the tolerance and the production cost. As can be seen, very small toler-
ances necessitate very high production cost. Therefore, small tolerances should not be
specified when designing a component unless they serve a certain purpose in that design.

Fits

Before two components are assembled together, the relationship between the dimensions
of the mating surfaces must be specified. In other words, the location of the zero line (i.e.,
the line indicating the basic size) to which deviations are referred must be established for
each of the two mating surfaces. As can be seen in Figure 1 .2a, this determines the degree
of tightness or freedom for relative motion between the mating surfaces. Figure 1 .2a also
shows that there are basically three types of fits: clearance, transition, and interference.

In all cases of clearance fit, the upper limit of the shaft is always smaller than the
lower limit of the mating hole. This is not the case in interference fit, where the lower limit
of the shaft is always larger than the upper limit of the hole. The transition fit, as the name
suggests, is an intermediate fit. According to the ISO, the internal enveloped part is always

FIGURE 1.2

The two systems of fit
according to the ISO:

(a) shaft-basis system;

(b) hole-basis system

Basic size

(a)

(b)

Hole tolerance zone

Shaft tolerance zone

Overview

referred to as the shaft, whereas the surrounding surface is referred to as the hole. Accord-
ingly, from the fits point of view, a key is the shaft and the key way is the hole.

It is clear from Figures 1.2a and b that there are two ways for specifying and
expressing the various types of fits: the shaft-basis and the hole-basis systems. The
location of the tolerance zone with respect to the zero line is indicated by a letter,
which is always capitalized for holes and lowercased for shafts, whereas the tolerance
grade is indicated by a number, as previously explained. Therefore, a fit designation
can be H7/h6, F6/g5, or any other similar form.

Interchangeability

When the service life of an electric bulb is over, all you do is buy a new one and re-
place the bulb. This easy operation, which does not need a fitter or a technician, would
not be possible without the two main concepts of interchangeability and standardiza-
tion. Interchangeability means that identical parts must be able to replace each other,
whether during assembly or subsequent maintenance work, without the need for any
fitting operations. Interchangeability is achieved by establishing a permissible toler-
ance, beyond which any further deviation from the nominal dimension of the part is
not allowed. Standardization, on the other hand, involves limiting the diversity and
total number of varieties to a definite range of standard dimensions. An example is the
standard gauge system for wires and sheets. Instead of having a very large number of
sheet thicknesses in steps of 0.001 inch, the number of thicknesses produced is limited
to only 45 (in U.S. standards). As you can see from this example, standardization has
far-reaching economic implications and also promotes interchangeability. Obviously,
the engineering standards differ for different countries and reflect the quality of tech-
nology and the industrial production in each case. Germany established the DIN
(Deutsche Ingenieure Normen), standards that are finding some popularity worldwide.
The former Soviet Union adopted the GOST, standards that were suitable for the pe-
riod of industrialization of that country.

THE PRODUCTION TURN

In almost all cases, the main goal of a manufacturing project is to make a profit, the ex-
ception being projects that have to do with the national security or prestige. Let us es-
tablish a simplified model that illustrates the cash flow through the different activities
associated with manufacturing so that we can see how to maximize the profit. As shown
in Figure 1.3, the project starts by borrowing money from a bank to purchase machines
and raw materials and to pay the salaries of the engineers and other employees. Next, the
raw materials are converted into products, which are the output of the manufacturing do-
main. Obviously, those products must be sold (through the marketing department) in
order to get cash. This cash is, in turn, used to cover running costs, as well as required
payment to the bank; any surplus money left is the profit.

We can see in this model that the sequence of events forms a continuous cycle (i.e.,
a closed circuit). This cycle is usually referred to as the production turn. We can also re-
alize the importance of marketing, which ensures the continuity of the cycle. If the prod-

1.7 Product Life Cycle

FIGURE 1.3

Initial money

The production turn

Bank

borrowed from bank

Manufacturing
plant

Products

Money to purchase

raw materials and

for the running cost

Money

payment
+ profit

Marketing
department

ucts are not sold, the cycle is obviously interrupted. Moreover, we can see that maxi-
mum profit is obtained through maximizing the profit per turn and/or increasing the
number of turns per year (i.e., running the cycle faster). Evidently, these two conditions
are fulfilled when products are manufactured in the easiest and least expensive way.

1.7 PRODUCT LIFE CYCLE

It has been observed that all products, from the sales viewpoint, go through the same
product life cycle, no matter how diverse or dissimilar they are. Whether the product
is a new-model airplane or a coffeemaker, its sales follow a certain pattern or sequence
from the time it is introduced in the market to the time it is no longer sold. The main
difference between the cycles of these two products is the span or duration of the
cycle, which always depends upon the nature and uses of the particular product. As we
will see later when discussing concurrent engineering in Chapter 2, it is very important
for the designer and the manufacturing engineer to fully understand that cycle in order
to maximize the profits of the production plant.

It is clear from Figure 1 .4 that the sales, as well as the rate of increase in sales, are ini-
tially low during the introduction stage of the product life cycle. The reason is that the con-
sumer is not aware of the performance and the unique characteristics of the product.

FIGURE 1.4

The product life cycle

Decline

Time

Overview

Through television and newspaper advertisements and word-of-mouth communication, a
growing number of consumers learn about the product and its capabilities. Meanwhile,
the management works on improving the performance and eliminating the shortcomings
through minor design modifications. It is also the time for some custom tailoring of the
product for slightly different customer needs, in order to serve a wider variety of con-
sumers. As a result, the customer acceptance is enhanced, and the sales accordingly in-
crease at a remarkable rate during this stage, which is known as the growth stage.
However, this trend does not continue forever, and, at a certain point, the sales level out.
This is, in fact, the maturity stage of the life cycle. During this stage, the product is usu-
ally faced with fierce competition, but the sales will continue to be stable if the manage-
ment succeeds in reducing the cost of the product and/or developing new applications for
it. The more successful the management is in achieving this goal, the longer the duration
of the maturity stage will be. Finally, the decline stage begins, the sales fall at a noticeable
rate, and the product is, at some point, completely abandoned. The decrease in the sales is
usually due to newer and better products that are pumped into the market by competing
manufacturers to serve some customer need. It can also be caused by diminishing need for
the uses and applications of such a product. A clever management would start developing
and marketing a new product (B) during the maturity stage of the previous one (A) so as
to keep sales continuously high, as shown in Figure 1.5.

FIGURE 1.5

The proper overlap of
products' life cycles

Time

1.8 TECHNOLOGY DEVELOPMENT CYCLE

Every now and then, a new technology emerges as result of active research and devel-
opment (R & D) and is then employed in the design and manufacture of several differ-
ent products. It can, therefore, be stated that technology is concerned with the industrial
and everyday applications of the results of the theoretical and experimental studies that
are referred to as engineering. Examples of modern technologies include transistor, mi-
crochip, and fiber optics.

The relationship between the effectiveness or performance of a certain technology
and the effort spent to date to achieve such performance is shown graphically in Fig-

1.8 Technology Development Cycle 9

ure 1.6. This graphical representation is known as the technology development cycle.
It is also sometimes referred to as the S curve because of its shape. As can be seen in
Figure 1.6, a lot of effort is required to produce a sensible level of performance at the
early stage. Evidently, there is a lack of experimental experience since the techniques
used are new. Next, the rate of improvement in performance becomes exponential, a
trend that is observed with almost all kinds of human knowledge. At some point, how-
ever, the rate of progress becomes linear because most ideas are in place; any further
improvement comes as a result of refining the existing ideas rather than adding new
ones. Again, as time passes, the technology begins to be "exhausted," and performance
levels out. A "ceiling" is reached, above which the performance of the existing current
technology cannot go because of social and/or technological considerations.

An enlightened management of a manufacturing facility would allocate resources
and devote effort to an active R&D program to come up with a new technology (B) as
soon as it realized that the technology on which the products are based (A) was beginning
to mature. The production activities would then be transferred to another S curve, with a
higher ceiling for performance and greater possibilities, as shown in Figure 1 .7. Any delay
in investing in R & D for developing new technology may result in creating a gap between
the two curves (instead of continuity with the overlap shown in Figure 1.7), with the final
outcome being to lose the market to competing companies that possess newer technology.
In fact, the United States dominated the market of commercial airliners because compa-
nies like Boeing and McDonnell Douglas knew exactly when to switch from propeller-
driven airplanes to jet-propulsion commercial airliners. This is contrary to what some
major computer companies did when they continued to develop and produce mainframe
computers and did not recognize when to make the switch to personal computers. Current
examples of technological discontinuity include the change from conventional telecom-
munications cables to fiber optics for communication and information transfer.

FIGURE 1.6

The technology
development cycle
(or S curve)

Effort

1 Overview

FIGURE 1.7

Transfer from one S
curve to another

Effort

THE DESIGN PROCESS

An engineer is a problem solver who employs his or her scientific and empirical
knowledge together with inventiveness and expert judgment to obtain solutions for
problems arising from societal needs. These needs are usually satisfied by some phys-
ical device, structure, or process. The creative process by which one or more of the
fruits of the engineer's effort are obtained is referred to as design. It is, indeed, the core
of engineering that provides the professional engineer with the chance of creating orig-
inal designs and watching them become realities. The satisfaction that the engineer
feels following the implementation of his or her design is the most rewarding experi-
ence in the engineering profession. Because design is created to satisfy a societal need,
there can be more than one way to achieve that goal. In other words, several designs
can address the same problem. Which one is the best and most efficient design? Only
time will tell because it is actually the one that would be favored by the customers
and/or the society as a whole.

Although there is no single standard sequence of steps to create a workable de-
sign, E. V. Krick has outlined the procedure involved in the design process, and his
work has gained widespread acceptance. Following is a discussion of the stages of the
design process according to Krick. (See the references at the back of the book for more
detailed information.)

Problem Formulation

As illustrated in Figure 1.8, problem formulation is the first stage of the design
process. This phase comes as a result of recognizing a problem and involves defining
that problem in a broad perspective without getting deep into the details. It is also at
this stage that the engineer decides whether or not the problem at hand is worth solv-
ing. In other words, this stage basically constitutes a feasibility study of the problem
arising from a recognized need. The designer should, therefore, realize the importance

1.9 The Design Process

FIGURE 1.8

The design process
(Adapted from Krick, An
Introduction to
Engineering and
Engineering Design,
2nd ed. New York: John
Wiley, 1969)

Recognition of
a problem to
be solved

Problem formulation
Problem analysis
Search
Decision
Documentation

Completely

specified

solution

The process
of design

of this stage. Neglecting it may result in wasting money in an effort to solve a prob-
lem that is not worth solving or wasting time on details that make it extremely difficult
to get a broad view of the problem so as to select the appropriate path for solving it.
The formulation of a problem can take any form that is convenient to the designer, al-
though diagrammatic sketching (in particular, the black-box method) has proven to be
a valuable tool.

Problem Analysis

The second stage involves much information gathering and processing in order to
come up with a detailed definition of the problem. Such information may come from
handbooks, from manufacturers' catalogs, leaflets, and brochures, as well as from per-
sonal contacts. You are strongly advised to seek information wherever you can find it;
workers at all levels of a company may have some key information that you can use.
The end product should be a detailed analysis of the qualitative and quantitative char-
acteristics of the input and output variables and constraints, as well as the criteria that
will be used in selecting the best design.

Search for Alternative Solutions

In the third stage, the designer actively seeks alternative solutions. A good practice is to
make a neat sketch for each preliminary design with some notes about its pros and cons.
All sketches should be kept even after a different final design is selected so that if that

12 1 Overview

final design is abandoned for some reason, a designer does not have to start from the be-
ginning again. It is also important to remember not to end the search for alternative so-
lutions prematurely, before it is necessary or desirable to do so. Sometimes, a designer
gets so involved with details of what he or she thinks is a good idea or solution that he
or she will become preoccupied with these details, spending time on them instead of
searching for other good solutions. Therefore, you are strongly advised to postpone
working out the details until you have an appropriate number of viable solutions.

It is, indeed, highly recommended to employ collaborative methods for enabling
the mind to penetrate into domains that might otherwise remain unexplored. A typical
example is the technique of brainstorming, where a few or several people assemble to
produce a solution for a problem by creating an atmosphere that encourages everyone
to contribute with whatever comes to mind. After the problem is explained, each mem-
ber comes up with an idea that is, in turn, recorded on a blackboard, thus making all
ideas evident to all team members.

Decision Making

The fourth stage involves the thorough weighing and judging of the different solutions
with the aim of being able to choose the most appropriate one. That is, trade-offs have
to be made during this stage. They can be achieved by establishing a decision matrix,
as shown in Figure 1 .9.

As can be seen in Figure 1 .9, each of the major design objectives is in a column, and
each solution is allocated a row. Each solution is evaluated with regard to how it fulfills
each of the design objectives and is, therefore, given a grade (on a scale of 1 to 10) in each
column. Because the design objectives do not have the same weight, each grade must be
multiplied by a factor representing the weight of the design function for which it was
given. The total of all the products of multiplication is the score of that particular solution
and can be considered as a true indication of how that solution fulfills the design objec-
tives. As you can see, this technique provides a mechanism for rating the various solu-
tions, thus eliminating most and giving further consideration to only a few.

The chosen design is next subjected to a thorough analysis in order to optimize
and refine it. Detailed calculations, whether manual or computational, are involved at
this point. Both analytical and experimental modeling are also extensively employed
as tools in refining the design. It is important, therefore, to now discuss modeling and
simulation. A model can be defined as a simplified representation of a real-life situa-
tion that aids in the analysis of an associated problem. There are many ways for clas-
sifying and identifying models. For example, models can be descriptive, illustrating a
real-world counterpart, or prescriptive, helping to predict the performance of the actual
system. They can also be deterministic or probabilistic (used when making decisions
under uncertainty). A simple example of a model is the free-body diagram used to de-
termine the internal tensile force acting in a wire with a weight attached to its end.
There are many computer tools (software) that are employed by designers to create
models easily and quickly. Examples include geometric modeling and finite element
analysis software packages. On the other hand, simulation can be defined as the
process of experimenting with a model by subjecting it to various values of input pa-

1.9 The Design Process

■ 2

H '«

ui I

a I

— to

u. <

/•

<B cu

o o

1- 1-

| 8

oo oS

O </>

+•

CO 00

■H T-i

CN CN

d d

O +3

_ O

q q

oo oJ

■H

•* h-

CN CN

CO CO

d d

i £

_ o

o o

oo oS

ir>

o in

«fr <fr

in in

c
o

5-2

d d

o ■*■>

_ o

o o

oo oS

. 3

•H

CN CO

5£

c c

E
5

op
w
a)

GO OB
CO en

Overview

rameters and observing the output, which can be taken as an indication of the behav-
ior of the real-world system under the tested conditions. As you can see, simulation
can save a lot of time and effort that could be spent on experimental models and pro-
totypes. This saving is particularly evident when computer simulation is employed.
Still, simulation would not eliminate design iterations but rather would minimize their
number. You are, therefore, urged to make use of these tools whenever possible.

Documentation

In the fifth and last stage of the design process, the designer organizes the material ob-
tained in the previous stage and puts it in shape for presentation to his or her superiors. The
output of this stage should include the attributes and performance characteristics of the re-
fined design, given in sufficient detail. Accordingly, the designer must communicate all
information in the form of clear and easy-to-understand documents. Documentation con-
sists of carefully prepared, detailed, and dimensioned engineering drawings (i.e., assem-
bly drawings and workshop drawings or blueprints), a written report, and possibly an
iconic model. With the recent development in rapid prototyping techniques, a prototype
can certainly be a good substitute for an iconic model. This approach has the advantage of
revealing problems that may be encountered during manufacturing.

1.10 PRODUCT DESIGN: THE CONCEPT
OF DESIGN FOR MANUFACTURING

The conventional procedure for product design, as illustrated in Figure 1.10, used to
start with an analysis of the desired function, which usually dictated the form as well
as the materials of the product to be made. The design (blueprint) was then sent to the
manufacturing department, where the kind and sequence of production operations
were determined mainly by the form and materials of the product. In fact, the old de-
sign procedure had several disadvantages and shortcomings:

FIGURE 1.10

The old procedure for
product design

Function

Form

Material

Feedback

Manufacturing

1.10 Product Design: The Concept of Design for Manufacturing

FIGURE 1.11

The new concept of a
manufacturing system
for achieving rational
product designs

1. In some cases, nice-looking designs were impossible to make; in many other cases,
the designs had to be modified so that they could be manufactured.

2. Preparing the design without considering the manufacturing process to be carried out
and/or the machine tools available would sometimes result in a need for special-
purpose, expensive machine tools. The final outcome was an increase in the produc-
tion cost.

3. When the required production volume was large, parts had to be specially designed
to facilitate operations involved in mass production (such as assembly).

4. A group of different products produced by the same manufacturing process has
common geometric characteristics and features that are dictated by the manufactur-
ing process employed (forgings, for example, have certain characteristic design fea-
tures that are different from those of castings, extrusions, or stampings). Ignoring
the method of manufacturing during the design phase would undermine these char-
acteristic design features and thus result in impractical or faulty design.

Because of these reasons and also because of the trend of integrating the activi-
ties in a manufacturing corporation, the modern design procedure takes into consid-
eration the method of manufacturing during the design phase. As can be seen in
Figure 1.11, design, material, and manufacturing are three interactive, interrelated el-
ements that form the manufacturing system, whose prime inputs are conceptual prod-
ucts (and/or functions) and whose outputs are manufactured products. In fact, the
barriers and borders between the design and manufacturing departments are fading

Function

Manufacturing
system

Products

1 Overview

out and will eventually disappear. The tasks of the designer and those of the manu-
facturing engineer are going to be combined and done by the same person. It is,
therefore, the mission of this text to emphasize concepts like design for manufactur-
ing and to promote the systems approach for product design.

Review Questions

1. What is the definition of manufacturing?

2. Is there any relationship between the status of
manufacturing in a nation and the standard of
living of the people in that nation? Explain
why.

3. Explain the different approaches for obtaining a
desired shape and give examples of some man-
ufacturing processes that belong to each group.

4. List the different types of production and ex-
plain the main characteristics of each. Also
mention some suitable applications for each
type.

5. Explain the meaning of the term tolerance.

6. How do we scientifically describe the tightness
or looseness of two mating parts?

7. What concepts did Eli Whitney establish to en-
sure trouble-free running of the mass produc-
tion of multicomponent products?

8. What is meant by the production turn? What
role does marketing play in this cycle?

9. Using the concept of production turn, how can
we maximize the profits of a company by two
different methods?

Explain the stages involved in the life cycle of
a product.

What is the significance of the product life
cycle during the phase of planning for the pro-
duction of new products?

What is the S curve? Explain an American suc-
cess story in employing it.

Give some examples of transfer from one tech-
nology development curve to another.

What are the stages involved in the design
process? Explain each briefly.

What is meant by trade-offs? How can these be
achieved during the decision-making stage?

Explain the old approach for product design.
What are its disadvantages?

17. Explain the concept of design for manufactur-
ing. Why is it needed in modern industries?

14.

Chapter 2

ncurrent
gineering

INTRODUCTION

Concurrent engineering is a manufacturing philosophy that involves managing
the product development process with the aim of getting new products with the
highest quality at the best competitive price in the least time to the market. It
has proven to be a key factor for the survival and, more importantly, for the
prosperity of companies that are clever enough to adopt its methodology and
tools (Motorola and Hewlett-Packard are good examples). In fact, many compa-
nies can, in good faith, argue that they have been using this methodology in
one form or another for some time — consider the efforts of corporations like
Xerox, Hewlett-Packard, and Ford in the late 1970s to review and revise their
product design practices versus those of their foreign competitors. An impor-
tant milestone in the history of concurrent engineering is considered to be the
report issued in December 1988 by the Defense Advanced Research Projects
Agency (DARPA) as a result of a study to improve concurrency in the product de-
sign process, a study that lasted more than five years. Many professionals in
this field rightfully believe that the DARPA report is the true foundation for the
concept of concurrent engineering. Many terms were and still are (though to a
lesser degree) used to describe this methodology. Examples include team de-
sign, simultaneous engineering, and integrated product development. The term
concurrent engineering was first coined by the Institute for Defense Analysis
(IDA), which also provided the following definition:

Concurrent engineering is a systematic approach to integrated, concur-
rent design of products and their related processes, including manufac-
ture and support. This approach is intended to cause the developers,

18 2 Concurrent Engineering

from the outset, to consider all elements of the product life cycle from
concept through disposal, including quality, cost, schedule, and user re-
quirements.

A good way to understand this new concept and what it means is to compare
the product development process in the traditional engineering approach, which
is usually referred to as serial or sequential engineering, with the one in the con-
current engineering environment. In serial engineering, a team of qualified pro-
fessional engineers designs a product without much interaction with or input from
other departments within the corporation such as manufacturing, sales, or cus-
tomer service. A model or prototype is then fabricated in the prototype workshop
based on the documented design produced by the team. Note that the environ-
ment in the model shop is ideal and is different from the real one on the shop floor
during production. Weeks or even months after releasing the design, the testing
department receives the model and carries out acceptance tests to make sure
that the model conforms to the documented design and also meets the criteria
established and agreed upon for the functioning and performance of the prod-
uct. As you may expect, alterations and modification and/or revisions of the de-
sign are needed in most cases as a result of the absence of inputs from other
departments during the design process. As a consequence, revisions call for
new designs that, in turn, require the fabrication and testing of new prototypes.

This obviously time-consuming cycle may have to be repeated a few times
in order to achieve the desired goals. Such a cycle prolongs the new product
development process to various degrees. Depending upon the complexity of the
product and the number of iterations, the delay can be excessive, thus causing
damage to the marketing strategy and sales of the new product. It is, therefore,
clear that the absence of communication between the different departments
starting early in the initial phase of the design process and continuing through-
out that process would result in a larger number of design iterations and de-
lays in releasing the product to the market. On the contrary, in a concurrent
engineering environment, all relevant departments, such as design, manufac-
turing, R&D, and marketing, become involved and participate in the design
process from its very beginning. This interaction reduces to a large extent or
even eliminates design iterations, thus compressing the development cycle
with the final outcome of reducing the time-to-market for a new product. The

2.1 Reasons for Adopting Concurrent Engineering 19

products can, therefore, be turned over at a much faster pace. Bearing in mind
that the larger portion of profit occurs in the early part of the cycle for suc-
cessful products, it would consequently be possible to allow new products to
be retired at nearly their optimum profitability. Also, because customers are
consulted (through the marketing and customer service departments) early in
the product development process, the new product would most probably pene-
trate the market easily because it would correctly meet customers' expecta-
tions in terms of both function and quality. A good example is the case of the
Boeing 777 jetliner. During the initial development stage, Boeing called repre-
sentatives of its customers, including BOAC (the British-government-owned air-
lines), although Britain is a part of the consortium building the air bus. By doing
so, Boeing got the praise and the support of its customers all over the world
(including BOAC) in the form of orders of the new product under development.
More importantly, some serious modifications in the design were made in the
very early phase, thus saving a lot of money and effort if they had not been
done. The choice of the height of the wings above ground level is a clear ex-
ample. Looking at the initial conceptual design, the customers realized that the
wing height was too high to the extent that it would create difficulties during fu-
eling and would require the use of a special fueling truck. Boeing was promptly
advised to lower the level of the wings so that the currently used fueling trucks
could be easily and successfully employed.

2.1 REASONS FOR ADOPTING
CONCURRENT ENGINEERING

Modern manufacturing industries are facing many challenges, such as global competi-
tion and fast-changing consumer demands. These and other challenges call for the
adoption of the concurrent engineering methodology. Following is a list of some of the
challenges that can be successfully met by concurrent engineering:

1. Increasing product complexities that prolong the product development process and
make it more difficult to predict the impact of design decisions on the functionality
and performance of the final product.

2. Increasing global competitive pressures that result from the emerging concept of
reengineering (which enabled many Asian countries to produce extremely cost-
effective products because the cost of R & D in this case is almost zero). This def-
initely creates the need for a cost-effective product development cycle.

20 2 Concurrent Engineering

The need for rapid response to fast-changing consumer demands. This phenomenon
calls for the need to continuously listen to the "voice" of the consumer — one of the
solid principles of concurrent engineering methodology.

The need for shorter product life cycles. This phenomenon necessitates the intro-
duction of new products to the market at a very high pace — something that can
only be achieved by compressing the product development cycle. Consider the
changes that the old-fashioned mechanical typewriter has undergone. Its life cycle
was 20 to 30 years, which then decreased to 10 years for an electromechanical
typewriter and finally to only 18 months for a word processor. This example clearly
illustrates the need to solve the problem of time-to-market pressure.

Large organizations with several departments working on developing numerous
products at the same time. The amount of data exchanged between these depart-
ments is extremely large, and unless properly managed in a rational manner, the
flow and transfer of information is not fast or easy (i.e., a piece of information
needed by a certain department may be passed to another one). The final outcome
would certainly be delays in the process of product development and products that
will not appear in the market at the scheduled time.

New and innovative technologies emerging at a very high rate, thus causing the
new products to be technologically obsolete within a short period of time. This phe-
nomenon is particularly evident in the electronics industry, where the life cycle of
a typical product is in months (it used to be years during the 1980s). As a conse-
quence, new products must appear in the market at a very high pace — something
that definitely necessitates a shorter product development cycle.

2.2 BENEFITS OF

CONCURRENT ENGINEERING

The benefits of adopting concurrent engineering are numerous and positively affect the
various activities in a corporation. Following is a summary of the important ones:

1. Because the customer is consulted during the early product development process,
the product will appear on the market with a high level of quality and will meet the
expectations of the customer. The product introduction region (or start-up) of the
product life cycle (see Figure 1.4) will be very short. The sales volume will, there-
fore, attain maturity in a very short time. As a consequence, large revenues and
profits would be achieved during the early phase of the product life cycle. This is
very important because products become technologically obsolete very quickly as a
result of fast-emerging, innovative technology.

2. Adopting concurrent engineering will result in improved design quality, which is
measured by the number of design changes made during the first six months after
releasing a new product to the market. These design changes are extremely expen-
sive unless caught early during the product development process. The lower the

2.3 Factors Preventing the Adoption of Concurrent Engineering 21

number of these changes, the more robust the design of the product is. In a concur-
rent engineering environment, these design changes would evidently be minimal.

3. Reduced product development and design times will result from listening to the
voice of the customer and from transferring information between the various de-
partments involved, including those downstream. This benefit is, in fact, a conse-
quence of the reduction in the number of design iterations necessary to achieve
optimum product design. Another factor is forsaking sequential methods of product
development and replacing them with concurrent ones.

4. Reduced production cost is a consequence of the preceding two benefits — namely,
the reduction in the number of design changes after releasing the product and the
reduction in the time of the product development process. Reduced cost, of course,
provides a manufacturing company with a real advantage in meeting global com-
petitive pressures.

5. Elimination of delays when releasing the product to the market will guarantee a
good market share for the new product. Also, it has been proven that delays in re-
leasing the product will result in market loss of revenues.

6. As a result of the reduced design time and effort, new products will be pumped into
the market more frequently, which is, indeed, the advantage that Japanese au-
tomakers have over their American counterparts. They can produce more different
models, with smaller production volumes and shorter life cycles.

7. Increased reliability and customer satisfaction will result from delivering the prod-
uct "right the first time" and will also enhance the credibility of the manufacturing
company.

2.3 FACTORS PREVENTING THE ADOPTION
OF CONCURRENT ENGINEERING

Now, if adopting concurrent engineering results in all the benefits just listed, why isn't
it widely applied in all manufacturing corporations? The reason is that concurrent en-
gineering is based on a manufacturing philosophy that requires breaking barriers be-
tween departments and establishing multidisciplinary teams. This philosophy clearly
contradicts the authoritative culture that is currently dominant in the industrial estab-
lishment. The threat of loss of power and authority makes middle management and bu-
reaucrats resistant to the idea of implementing concurrent engineering. There is also a
natural resistance to anything new inherent in the minds of some people. Another fac-
tor may be the need to build an excellent communication infrastructure for facilitating
the flow of information throughout the product development cycle. Apparently, a lot of
money and effort must be invested to create an adequate information system — some-
thing that many companies either cannot afford or do not want to do. Yet another fac-
tor that holds up the implementation of concurrent engineering is the temptation to
come up with temporary short-run solutions to the problem of decreasing revenues,

22 2 Concurrent Engineering

without any regard to strategic planning and long-term goals. Examples include cutting
the work force to increase profits (a faulty, shortsighted approach that would cause a
company to lose trained employees who are needed to enable quick, on-schedule prod-
uct delivery) and cutting the price without any basis (a solution that would inevitably
eliminate or reduce the profits).

2.4 THE FOUR PILLARS

OF CONCURRENT ENGINEERING

The implementation of concurrent engineering is based on managing forces of change
and using them as resources or tools in four arenas for efficient, fast, and economical
product development. These arenas — organization, communication infrastructure, re-
quirements, and product development — are the pillars on which the methodology of
concurrent engineering rests. Let us examine each of them and see how each can be
managed.

Organization

This arena includes the managers, product development teams, and support teams (i.e.,
the organization itself and the interactions of its components). The role of management
is vital and includes not only motivating people to change their work habits to match
the concurrent engineering environment but also ensuring unhindered exchange of in-
formation between the different disciplines. In fact, management can be used as one of
the forces of change or tools that guarantees continuous improvement of the product
development process, as will be discussed later.

Communication Infrastructure

The communication infrastructure encompasses the hardware, software, and expertise
that together form a system that allows the easy transfer of information relating to
product development. As you may expect, when the product complexity increases, the
number of disciplines involved also increases, as does the volume of information to be
transferred. The system to be established must be capable of handling the type and
amount of data necessary for the product development process. It must retrieve, eval-
uate, and present the data in an organized format that is easy to understand and to use
by team members and by management. In fact, many corporations learned the hard
way that communication technologies are as important as design and manufacturing
technologies for the success of a new product. The first task to handle, after purchas-
ing the hardware, is to build a comprehensive and efficient database that has queries
and that can be accessed by teams and by managers who are in charge of monitoring
and evaluating the product development process. Electronic mail, interactive browsing
capabilities, and other modern information transfer technology are also essential in
order to eliminate the need for shoveling piles of documents and papers between the
different departments and teams.

2.4 The Four Pillars of Concurrent Engineering 23

Requirements

A broad (but accurate) description of the product requirements involves all product at-
tributes that affect customer satisfaction. Consequently, customers' needs are considered
when setting the specifications for the conceptual design. This consideration would, in-
deed, ensure that the model or prototype meets the original goals from the start. This
process of creating the conceptual specifications is extremely important and must be car-
ried out rigorously. The more product attributes and constraints are initially specified, the
fewer the problems associated with the final product design are, and the fewer the number
of design changes or iterations will be. Of course, the conceptual design constraints
should be defined very clearly and subjected to a continuous process of updating, evalua-
tion, and validation. As is well known, constraints like government regulations, envi-
ronmental laws, and industry and national standards are changing all the time.
Continually updating these would certainly improve the product development process.

Product Development

In a concurrent engineering environment, the downstream processes of manufacturing,
maintenance, customer service, sales, and so on, must be considered in the early design
phase. This consideration, as previously mentioned, is a necessary condition for the im-
plementation of concurrent engineering. The second important condition is the need for
continuous improvement and optimization of the product development process. As a
consequence of these two conditions, there is a continuous drive to develop, evaluate,
and adopt new design methodologies. Concepts and approaches like design for manu-
facturing (DFM) and design for assembly (DFA) have been popular in recent days and
have proven to be valuable tools in adopting and implementing concurrent engineering.
The reason is that their philosophy is based on using manufacturing (or assembly) as a
design constraint, thus taking downstream processes like manufacturing and assembly
into full consideration during the early design phase of the product. In other words, the
success of these methodologies is linked to their philosophies being compatible to (or
matching) that of concurrent engineering.

Another part of the product development arena is what is sometimes referred to as
the component libraries. The design (and manufacturing) attributes of the different
components, whether standard ones that were purchased or parts that were previously
designed and manufactured, are kept in a database. The availability of such a database
to team members will speed up the design process by providing them with many al-
ternatives to choose from and, more importantly, by freeing them from reinventing the
wheel. Using previously tested components, maybe with very slight modification in
the design, can save a lot of time and effort. Keeping a computerized database has the
advantages of easy retrieval of designs and simultaneous availability to all team mem-
bers. This topic will be covered later in the book in detail when we discuss group tech-
nology and computer-aided process planning.

Also part of the product development arena is the design process itself. We have
already covered its stages and methodology in Chapter 1. Here, we want to emphasize
again that good design has always been based on customer needs, which must be, in

Concurrent Engineering

turn, determined by listening to customer concerns. In fact, it was for this reason that
the concept of quality function deployment (QFD) was developed in Japan's Kyoto
Shipyard in the 1980s. By including QFD in the design process, teams do not lose
touch with the customer, and, consequently, the designed products will meet cus-
tomers' needs and expectations. Although QFD is beyond the scope of this text, a brief
discussion will enlighten those engineers who must communicate with members in
charge of QFD in a multidisciplinary team. QFD seeks to identify and evaluate the
meaning of the word quality from the customer's point of view. The approach involves
constructing a matrix that is quite similar to the design decision matrix covered in
Chapter 1 (see Figure 1.9). This matrix is called the house of quality. The attributes,
functions, and characteristics that the customer wants can be clearly identified and
used as input constraints or requirements for the process of designing the product as
previously mentioned. To repeat, remember that one of the goals of the design team is
to have a decreasing number of design changes with increasing order of design stage.
In the final design stage, if the appropriate methodology is adopted, the number of
changes should be zero.

ORCES OF CHANGE

Implementing concurrent engineering is based on managing some forces of change and
using them as resources or tools to create the concurrent engineering environment. Fol-
lowing are some of these forces of change.

Technology

Technology has a very important role to play in each of the four arenas of concurrent
engineering. It speeds up and optimizes the product development process, minimizes
the number of design iterations, and facilitates communication and information trans-
fer between the different teams and departments. Managers should, therefore, take full
advantage of the most up-to-date technology available and avoid technology that is
under development or obsolete. Unfortunately, the problem of acquiring up-to-date
technology is far more complicated than it seems because of the extremely fast pace
with which technology is advancing and the vast amount of different options available.
Here are some tips that address the technology problem:

1. Keep engineers abreast of the latest technological developments by providing them
with technical journals and periodicals, sending them to international engineering
conferences and exhibitions, and ensuring a continuous learning process through
workshops and short courses offered on site.

2. Ensure that the latest scientific findings are promptly employed in developing a com-
pany's technology and, therefore, result in high-quality products. Companies should
focus their efforts on applied research for developing products and processes and in-
tegrate their R&D with design and development activities. (In fact, this is one of the
reasons behind the success of several countries in the Far East.)

2.5 Forces of Change 25

3. Try to make use of the results of government-funded research and thus save time
and money spent in obtaining similar findings.

4. Definitely overcome the "not-invented-here" syndrome. Many industry people
make the mistake of completely ignoring any technology that was not invented in
their company. This syndrome leads to isolationism and, eventually, falling behind.
It is very difficult for a company to fully develop technology starting from scratch.
Acquiring technology by purchasing it, by establishing partnerships between com-
panies, and by encouraging technology exchanges is worth exploring.

It is important here to cast light on one of industry's most difficult problems in the
United States — the bad effects of having advanced technology geared toward military
applications. Although there is a wealth of technological information as a result of ac-
tive R & D in military industries, it is classified and, therefore, not accessible for civilian in-
dustries and commercial applications. A further obstacle is the difference in the product
requirements in both cases. Although military criteria specify quality regardless of cost,
civilian requirements call for both quality and cost. The picture is clear when you compare
the performance and the cost of a nuclear bomber with those of a commercial jetliner.

Management

Management in a concurrent engineering environment takes its role from management
in a traditional serial manufacturing corporation and goes far beyond it. A manager's
role involves not only setting schedules and work expectations of engineers and assign-
ing responsibilities but also managing changes and building an organizational structure
that is flexible and can respond quickly to surprises and sudden changes in demands and
requirements. You may have already concluded that managers must have a general but
solid understanding of current and relevant technical issues in order to communicate ef-
fectively with multidisciplinary teams. In fact, one of the most important tasks of mid-
dle management in a concurrent engineering environment is the creation of those
multidisciplinary teams in order to carry out the product development process. In sum-
mary, the traditional role of management that is based on vertical chain of command, au-
thoritative decision making, and the "carrot-stick" model of running corporations is
diminishing continuously, especially in a company that adopts the concurrent engineer-
ing philosophy. More emphasis is being placed on creating product development teams
and facilitating information transfer and communication between them.

Let us look more thoroughly at the process of establishing multidisciplinary teams.
As you may expect, complex tasks are handled by breaking them into less complex ones
that are, in turn, dealt with simultaneously but separately with different teams. Good man-
agers should optimize the size of each team. A team that is too large or too small creates
communication problems, is less efficient, and is more expensive. Attention must also be
given to the talents and the quality of team members in terms of choosing the right person
for the right job. When establishing the teams, the management focus must be to concur-
rently execute tasks that are normally carried out sequentially and to integrate those ac-
tivities that are concurrent. Consequently, an appropriate project-modeling tool must be
used in order to identify and locate the patterns of information flow and interaction. There

Concurrent Engineering

FIGURE 2.1

The PERT chart

are basically three approaches or tools — namely, the PERT chart, the GANTT chart, and
the design structure matrix (DSM).

The PERT chart, which is illustrated in Figure 2.1, is basically used to determine
project duration and critical path. On the other hand, the GANTT chart displays the
relative positioning of tasks on a time scale, as shown in Figure 2.2. In fact, some re-
searchers believe that the DSM method is far better in displaying the connectivity of
interacting tasks and improving the product development process. It also clearly illus-
trates where the integration of tasks should take place.

The DSM method and its modified version have been extensively used by Smith
and Eppinger of the Sloan School of Management at the Massachusetts Institute of
Technology (MIT). The basic method involves representing the relationship among
project tasks in a matrix form and allows for different tasks to be coupled. As can be
seen in Figure 2.3, each individual task is represented by a row and by a column of a
square matrix; the need for information flow between two tasks is indicated by a check
mark (x). Going horizontally across a task's row, the columns under which there are
check marks are those from which information must be received in order to complete
the given task. On the other hand, going vertically down a task's column, the check

FIGURE 2.2

The GANTT chart

Task A

/ '

TaskB

TaskC

Task D

2.5 Forces of Change

FIGURE 2.3

Initial phase of the
design structure matrix

Tasks ABCDEFGH I
A
B
C

E
F
G
H

KLMNOPQRSTUV

marks indicate the rows (tasks) that require output from the given column. The diago-
nal elements are hatched because a task cannot be coupled with itself. Now, structur-
ing the teams, usually referred to as product development teams (PDTs), can be
accomplished by identifying highly coupled sets of tasks. First, the rows and columns
must be rearranged so as to yield "batches" of check marks where a few tasks are cou-
pled together and where the information of PDTs is most appropriate, as shown in Fig-
ure 2.4. The process of swapping the rows and columns of the matrix requires
experience and skill because it is based on trial and error. Nevertheless, the process is
also amenable to computer manipulation and analysis. As you can see, however, the
DSM model does not take into consideration the degree of dependence or coupling be-
tween each two tasks. Recently, Smith and Eppinger replaced the check marks by
numbers indicating the strength of dependence. The eigenvalue of such a matrix would
reveal the highly coupled sets of tasks.

After the teams are established, the next question is how to manage the product
development project and ensure that it is on target to meet the previously agreed-upon
milestones and deadlines. Again, PERT and GANTT charts can be employed to

Concurrent Engineering

FIGURE 2.4

Final phase of the
design structure matrix

Tasks A F G D E
A
F
G
D
E
I

B
C
J
K
P
H
N
O
Q
L
M
R
S
T
U
V

BCJ KPHNOQLMRSTUV

achieve these goals. There are, however, some other methods for project updating that
visually illustrate the project status in one integrated chart, thus ensuring that different
PDTs meet their stated goals concurrently. The radar (spider) chart is a popular one. As
can be seen in Figure 2.5, each task or area of activity is represented by a radial line
and for a one-year period. Thus, one look at the chart is enough to see whether or not
the separate goals in the different areas are met. In the ideal case, when all tasks are
performed exactly according to the planned schedule, this chart will end up having
concentric circles at the various time periods, as indicated by the dashed lines in Fig-
ure 2.5. On the other hand, the bug chart is a plot of project expenditures versus prod-
uct goals or milestones, which are indicated on the time axis as shown in Figure 2.6.
The scales of both axes are adjusted so that a project that is on track is represented by
a straight line making 45° with both axes. Although this chart has the advantages of in-
dicating project-cost updating and individual milestones, it is sometimes misleading (a
delay in purchasing supplies, for example, might seem or be interpreted as a positive
indication). Further details are beyond the scope of this text, and interested readers are
advised and encouraged to seek specialized books on the subject.

2.5 Forces of Change

FIGURE 2.5

The radar (spider) chart

Area of activity

FIGURE 2.6

The bug chart

Manufacturing
release
exenditure
target

Actual

manufacturing
release date
and expenditure

Milestone

Manufacturing release
date target

30 2 Concurrent Engineering

Tools

There is an extremely large number of tools for handling various tasks in the different
arenas. The selection of the right tool for the right job is, therefore, not easy. In addi-
tion, most of these tools are undergoing a rapid, continuous, never-ending evolution.
Tools that were new in the 1980s are now technically obsolete, which indicates the
need to continually upgrade and replace a company's acquired tools. For example,
structured analysis software tools manage information systems and present complex
systems in a clear, easy-to-comprehend way. Instead of the old-fashioned written flow-
charting procedures, this software provides graphical representation of any complex
operation by a network of elements (each stands for a particular function) and by ar-
rows indicating data flow and interaction between those elements. It also enables the
elimination of redundant loops, thus making an operation more efficient.

Another example is tools used for design automation. They find increasing appli-
cation in manufacturing corporations, and it is anticipated that by the year 2000 about
80 percent of all designs will be electronically done using these tools. Also, integration
of these islands of automation (i.e., engineering departments in a manufacturing firm)
is the trend in the 1990s, where local-area networks (LANs) are extensively used to
transfer information from one department to another in a standardized format.

The adoption of new tools creates the problem of changing the responsibilities and
nature of the jobs of employees, who will need retraining and, sometimes, have to be
swapped. Also, based on the preceding discussion, the level of automation used must
be appropriate for the company, and automated PDTs must be integrated into a system.

It is always important to remember that the forces of change discussed herein are
just examples and that there can be other forces of change depending upon the nature of
the manufacturing corporation and its production. Nevertheless, in all cases and regard-
less of the tools used, the change from serial manufacturing to concurrent engineering
must be well planned and managed so as to take place gradually and smoothly and must
always be monitored by management. In fact, abrupt changes and employee dissatisfac-
tion are two factors that can impede the implementation of concurrent engineering.

> ~

J.6 A SUCCESS STORY: NIPPONDENSO

Now that you understand the arenas of concurrent engineering and the forces of
change, it is time to look at a case study indicating how concurrent engineering was
successfully implemented and resulted in solving tough problems that were facing one
of the world's largest manufacturers of automotive parts. The original report was given
in a paper entitled "Nippondenso Co. Ltd: A Case Study of Strategic Product Design,"
authored by Daniel E. Whitney and presented at the Collaborative Engineering Con-
ference held at MIT in October 1993. This paper contained a wealth of information
and was based on seven personal visits by the author to Nippondenso Co. Ltd. during
the period 1974 to 1991, as well as on interviews with the company's personnel and
papers published by the engineering staff. The information has been rearranged here,
however, so as to draw parallelism with the previously mentioned concurrent engi-
neering model and its four arenas.

2.6 A Success Story: Nippondenso 31

Nippondenso Co. Ltd. is one of the world's largest manufacturers of automotive
components, including air conditioners, heaters, relays, alternators, radiators, plus me-
ters, diesel components, filters, controls, brake systems, and entertainment equipment.
The company has 20 plants in 15 foreign countries in addition to 10 plants in Japan. In
1991, almost 43,000 people were employed by the company worldwide. Nippondenso
is the first-tier supplier to Toyota and other Japanese and foreign car companies, and
its sales amounted to about $10 billion dollars in 1989. Now that you have a clear idea
about the size of this company and the diversity of its products, let us see how they
created a concurrent engineering environment. Following is the company's approach
in each of the previously mentioned arenas.

Organization

Nippondenso's philosophy is based on developing the product and the process for mak-
ing it simultaneously. Consequently, multidisciplinary teams are formed through repre-
sentation from various departments like production engineering, machines and tools,
product design, and so on. Teams are small at the beginning but become larger as the
project proceeds from the concept phase to the detailed-design phase. Top management
promptly steps in when a crisis occurs and when a crucial decision needs to be made. Of
course, a parallel-task approach is employed by overlapping some of the design steps.

Requirements

In addition to product performance specifications and production cost targets, there are
other severe constraints dictated by the nature of the business of Nippondenso as a
supplier to large auto manufacturers (i.e., the need to meet ordering patterns). The re-
quirements of customers (like Toyota) include delivering extremely large amounts of
products on a just-in-time (JIT) basis, with high variety and an unpredictable model
mix that is always changing. A further constraint is to achieve all these goals with lit-
tle or no changeover time. As you will see later, defining customer requirements
helped Nippondenso to address the problems in a rational, thoughtful manner.

Communication Infrastructure

Nippondenso built an excellent system for information exchange. It is used to integrate
the different machine tools throughout the plant through local- or wide-area networks
that are, in turn, linked with the engineering departments dealing with computer con-
trol, scheduling, quality monitoring, and the like. Any change in data by a team mem-
ber is promptly made available to all members of other teams, thus breaking the
barriers between departments and between teams.

Product Development

The two most important elements upon which Nippondenso's approach in the product
development arena is based include developing the product and its manufacturing
processes simultaneously and developing new product design methodologies. In fact,
this approach is credited for enabling Nippondenso to meet customer requirements.

Concurrent Engineering

In order to meet the challenge of high production volume and high variety, the
first step for Nippondenso was standardization after negotiating with customers and
listening to their concerns. The next step was to design the products intelligently so as
to achieve the desired flexibility during assembly, rather than employing complex and
expensive production methods. In other words, their philosophy was based on using
assembly rather than manufacturing to make different models. High variety was
achieved by producing several versions of each component in the product and then as-
sembling the appropriate component's versions into any desired model. Thus, an ex-
tremely large number of combinations of component versions resulted in a large
number of possible models. Moreover, this approach also ensured quick changeover
from one model to another.

At this point, the basic concept of concurrent engineering has been thoroughly
demonstrated. Interested readers are encouraged to consult more specialized books on
the subject (see the titles provided in the references at the back of the book).

>w Questions

1. Define the term concurrent engineering and
elaborate on its meaning.

2. In what way does a concurrent engineering en-
vironment differ from that of serial manufactur-
ing?

3. How did concurrent engineering come into
being?

4. What are the reasons for adopting concurrent
engineering?

5. Discuss three of these reasons in detail.

6. List the benefits of adopting concurrent engi-
neering and discuss three of them in detail.

7. If concurrent engineering is so beneficial, why
don't all manufacturing companies adopt it?

10.

11.

What are the four pillars on which concurrent
engineering rests?

What is the difference between the role of man-
agement in a concurrent engineering environ-
ment and that role in conventional serial
manufacturing?

How does the product development process dif-
fer in a concurrent engineering environment
from that in conventional serial manufacturing?

Explain why new concepts like DFM, DFA,
and QFD are important and very useful when
implementing concurrent engineering.

INTRODUCTION

Definition. The word casting is used both for the process and for the product.
The process of casting is the manufacture of metallic objects (castings) by
melting the metal, pouring it into a mold cavity, and allowing the molten metal
to solidify as a casting whose shape is a reproduction of the mold cavity. This
process is carried out in a foundry, where either ferrous (i.e., iron-base) or non-
ferrous metals are cast.

Casting processes have found widespread application, and the foundry in-
dustry is considered to be the sixth largest in the United States because it pro-
duces hundreds of intricately shaped parts of various sizes like plumbing
fixtures, furnace parts, cylinder blocks of automobile and airplane engines, pis-
tons, piston rings, machine tool beds and frames, wheels, and crankshafts. In
fact, the foundry industry includes a variety of casting processes that can be
classified in one of the following three ways:

1. By the mold material and/or procedure of mold production

2. By the method of filling the mold

3. By the metal of the casting itself

Historical Background. At the dawn of the metal age, human knowledge was not
advanced enough to generate the high temperatures necessary for smelting
metals. Therefore, because casting was not possible, metals were used as
found or heated to a soft state and worked into shapes. The products of that era
are exemplified by the copper pendant from Shanidar Cave (northeast of Iraq),
which dates back to 9500 b.c. and which was shaped by hammering a piece of

34 3 Casting and Foundry Work

native metal and finishing with abrasives. Later, copper-smelting techniques
were developed, and copper castings were produced in Mesopotamia as early as
3000 b.c. The art of casting was then refined by the ancient Egyptians, who in-
novated the "lost-wax" molding process. During the Bronze Age, foundry work
flourished in China, where high-quality castings with intricate shapes could be
produced. The Chinese developed certain bronze alloys and mastered the lost-
wax process during the Shang dynasty. Later, that art found its way to Japan with
the introduction of Buddhism in the sixth century. There were also some signifi-
cant achievements in the West, where the Colossus of Rhodes, a statue of the
sun god Helios weighing 360 tons, was considered to be one of the seven won-
ders of the world. That bronze statue was cast in sections, which were assem-
bled later, and stood 105 feet high at the entrance of the harbor of Rhodes.

Although iron was known in Egypt as early as 4000 b.c, the development of
cast iron was impossible because the high melting temperature needed was not
achievable then and pottery vessels capable of containing molten iron were not
available. The age of cast iron finally arrived in 1340 when a flow oven (a crude
version of the blast furnace) was erected at Marche-Les-Dames in Belgium. It was
capable of continuous volume production of molten iron. Ferrous foundry practice
developed further with the invention of the cupola furnace by John Wilkenson in
England. This was followed by the production of black-heart malleable iron in
1826 by Seth Boyden and the development of metallography by Henry Sorby of
England. The relationship between the properties and the microstructure of alloys
became understood, and complete control of the casting process became feasi-
ble based on this knowledge. Nevertheless, forming processes developed more
rapidly than foundry practice because wrought alloys could better meet a wider
range of applications. Nodular cast iron, which possesses both the castability of
cast iron and the impact strength of steel, was introduced in 1948, thus paving
the way for castings to compete more favorably with wrought alloys.

3.1 CLASSIFICATIONS OF CASTING
BY MOLD MATERIAL

Molds can be either permanent or nonpermanent. Permanent molds are made of steel,
cast iron, and even graphite. They allow large numbers of castings to be produced suc-
cessively without changing the mold. A nonpermanent mold is used for one pouring

3.1 Classifications of Casting by Mold Material 35

only. It is usually made of a silica sand mixture but sometimes of other refractory ma-
terials like chromite and magnesite.

Green Sand Molds

Molding materials. Natural deposits taken from water or riverbeds are used as mold-
ing materials for low-melting-point alloys. Thus, the material is called green sand,
meaning unbaked or used as found. These deposits have the advantages of availability
and low cost, and they provide smooth as-cast surfaces, especially for light, thin jobs.
However, they contain 15 to 25 percent clay, which, in turn, includes some organic im-
purities that markedly reduce the fusion temperatures of the natural sand mixture,
lower the initial binding strength, and require a high moisture content (6 to 8 percent).
Therefore, synthetic molding sand has been developed by mixing a cleaned pure silica
sand base, in which grain structure and grain-size distribution are controlled, with up
to 18 percent combined fireclay and bentonite and only about 3 percent moisture. Be-
cause the amount of clay used as a binding material is minimal, synthetic molding sand
has higher refractoriness, higher green (unbaked) strength, better permeability, and
lower moisture content. The latter advantage results in the evolution of less steam dur-
ing the casting process. Thus, control of the properties of the sand mixture is an im-
portant condition for obtaining good castings. For this reason, a sand laboratory is
usually attached to the foundry to determine the properties of molding sands prior to
casting. Following are some important properties of a green sand mixture:

1. Permeability. Permeability is the most important property of the molding sand and
can be defined as the ability of the molding sand to allow gases to pass through.
This property depends not only on the shape and size of the particles of the sand
base but also on the amount of the clay binding material present in the mixture and
on the moisture content. The permeability of molds is usually low when casting
gray cast iron and high when casting steel.

2. Green compression strength of a sand mold. Green strength is mainly due to the
clay (or bentonite) and the moisture content, which both bind the sand particles to-
gether. Molds must be strong enough not to collapse during handling and transfer
and must also be capable of withstanding pressure and erosion forces during pour-
ing of the molten metal.

3. Moisture content. Moisture content is expressed as a percentage and is important
because it affects other properties, such as the permeability and green strength. Ex-
cessive moisture content can result in entrapped steam bubbles in the casting.

4. Flowability. Flowability is the ability of sand to flow easily and fill the recesses and
the fine details in the pattern.

5. Refractoriness. Refractoriness is the resistance of the molding sand to elevated
temperatures; that is, the sand particles must not melt, soften, or sinter when they
come in contact with the molten metal during the casting process. Molding sands
with poor refractoriness may burn when the molten metal is poured into the mold.
Usually, sand molds should be able to withstand up to 3000°F (1650°C).

36 3 Casting and Foundry Work

Sand molding tools. Sand molds are made in flasks, which are bottomless containers.
The function of a flask is to hold and reinforce the sand mold to allow handling and
manipulation. A flask can be made of wood, sheet steel, or aluminum and consists of
two parts: an upper half called the cope and a lower half called the drag. The standard
flask is rectangular, although special shapes are also in use. For proper alignment of
the two halves of the mold cavity when putting the cope onto the drag prior to casting,
flasks are usually fitted with guide pins. When the required casting is high, a middle
part, called the cheek, is added between the drag and the cope. Also, when a large
product is to be cast, a pit in the ground is substituted for the drag; the process is then
referred to as pit molding.

Other sand molding tools can be divided into two main groups:

1. Tools (such as molders, sand shovels, bench rammers, and the like) used for fill-
ing the flask and ramming the sand

2. Tools (such as draw screws, draw spikes, trowels, slicks, spoons, and lifters) used
for releasing and withdrawing the pattern from the mold and for making required
repairs on or putting finishing touches to the mold surfaces

Patterns for sand molding. The mold cavity is the impression of a pattern, which is
an approximate replica of the exterior of the desired casting. Permanent patterns
(which are usually used with sand molding) can be made of softwood like pine, hard-
wood like mahogany, plastics, or metals like aluminum, cast iron, or steel. They are
made in special shops called pattern shops. Wood patterns must be made of dried or
seasoned wood containing less than 10 percent moisture to avoid warping and dis-
tortion of the pattern if the wood dries out. They should not absorb any moisture
from the green molding sand. Thus, the surfaces of these patterns are painted and
coated with a waterproof varnish. A single-piece wood pattern can be used for mak-
ing 20 to 30 molds, a plastic pattern can be used for 20,000 molds, and a metal pat-
tern can be used for up to 100,000 molds, depending upon the metal of the pattern.
In fact, several types of permanent patterns are used in foundries. They include the
following:

1. Single or loose pattern. This pattern is actually a single copy of the desired cast-
ing. Loose patterns are usually used when only a few castings are required or when
prototype castings are produced.

2. Gated patterns. These are patterns with gates in a runner system. They are used to
eliminate the hand-cutting of gates.

3. Match-plate patterns. Such patterns are used for large-quantity production of
smaller castings, where machine molding is usually employed. The two halves of
the pattern, with the line of separation conforming to the parting line, are perma-
nently mounted on opposite sides of a wood or metal plate. This type of pattern al-
ways incorporates the gating system as a part of the pattern.

4. Cope-and-drag pattern plates. The function of this type of pattern is similar to that
of the match-plate patterns. Such a pattern consists of the cope and drag parts of the

3.1 Classifications of Casting by Mold Material 37

pattern mounted on separate plates. It is particularly advantageous for preparing
molds for large and medium castings, where the cope and drag parts of the mold are
prepared on different molding machines. Therefore, accurate alignment of the two
halves of the mold is necessary and is achieved through the use of guide and locat-
ing pins and bushings in the flasks.

In order for a pattern to be successfully employed in producing a casting having
the desired dimensions, it must not be an exact replica of the part to be cast. A number
of allowances must be made on the dimensions of the pattern:

1. Pattern drafts. This is a taper of about 1 percent that is added to all surfaces per-
pendicular to the parting line in order to facilitate removal of the pattern from the
mold without ruining the surfaces of the cavity. Higher values of pattern draft are
employed in the case of pockets or deep cavities.

2. Shrinkage allowance. Because molten metals shrink during solidification and con-
tract with further cooling to room temperature, linear dimensions of patterns must
be made larger to compensate for that shrinkage and contraction. The value of the
shrinkage allowance depends upon the metal to be cast and, to some extent, on
the nature of the casting. The shrinkage allowance is usually taken as 1 percent for
cast iron, 2 percent for steel, 1.5 percent for aluminum, 1.5 percent for magnesium,
1 .6 percent for brass, and 2 percent for bronze. In order to eliminate the need for
recalculating all the dimensions of a casting, pattern makers use a shrink rule. It is
longer than the standard 1-foot rule; its length differs for the different metals of the
casting.

3. Machine finish allowance. The dimensions on a casting are oversized to compen-
sate for the layer of metal that is removed through subsequent machining to obtain
better surface finish.

4. Distortion allowance. Sometimes, intricately shaped or slender castings distort dur-
ing solidification, even though reproduced from a defect-free pattern. In such cases,
it is necessary to distort the pattern intentionally to obtain a casting with the desired
shape and dimensions.

Cores and core making. Cores are the parts of the molds that form desired internal
cavities, recesses, or projections in castings. A core is usually made of the best quality
of sand to have the shape of the desired cavity and is placed into position in the mold
cavity. Figure 3.1 shows the pattern, mold, and core used for producing a short pipe
with two flanges. As you can see, projections, called core prints, are added to both
sides of the pattern to create impressions that allow the core to be supported and held
at both ends. When the molten metal is poured, it flows around the core to fill the rest
of the mold cavity. Cores are subjected to extremely severe conditions, and they must,
therefore, possess very high resistance to erosion, exceptionally high strength, good
permeability, good refractoriness, and adequate collapsibility (i.e., the rapid loss of
strength after the core comes in contact with the molten metal). Because a core is sur-
rounded by molten metal from all sides (except the far ends) during casting, gases have
only a small area through which to escape. Therefore, good permeability is sometimes

3 Casting and Foundry Work

FIGURE 3.1

The pattern, mold, and
core used for producing
a short pipe

Core

Pattern

(before removal

from mold)

assisted by providing special vent holes to allow gases to escape easily. Another re-
quired characteristic of a core is the ability to shrink in volume under pressure without
cracking or failure. The importance of this characteristic is obvious when you consider
a casting that shrinks onto the core during solidification. If the core is made hard
enough to resist the shrinkage of the casting, the latter would crack as a result of being
hindered from shrinking. Figure 3.2 is a photograph of a sand core for an automotive
cam tunnel.

Core sand is a very pure, fine-grained silica sand that is mixed with different
binders, depending upon the casting metal with which it is going to be used. The
binder used with various castings includes fireclay, bentonite, and sodium silicate (in-
organic binders), as well as oils (cottonseed or linseed oil), molasses, dexstrin, and
polymeric resins (organic binders).

Cores are usually made separately in core boxes, which involve cutting or ma-
chining cavities into blocks of wood, metal, or plastic. The surfaces of each cavity
must be very smooth, with ample taper or draft, to allow easy release of the green (un-
baked) core. Sometimes, a release agent is applied to the surfaces of the cavity. Core
sand is rammed into the cavity, and the excess is then struck off evenly with the top of
the core box. Next, the green core is carefully rolled onto a metal plate and is baked in
an oven. Intricate cores are made of separate pieces that are pasted together after bak-
ing. Sometimes, cores are reinforced with annealed low-carbon steel wires or even

FIGURE 3.2

Core for an automotive
cam tunnel

3.1 Classifications of Casting by Mold Material

FIGURE 3.3

A simple core and its
corresponding core box

Left half of
core box

Right half of
core box

Core

Vent hole

cast-iron grids (in the case of large cores) to ensure coherence and stability. Figure 3.3
illustrates a simple core and its corresponding core box.

Large, round cores can be made by means of sweeps or templates, and drawing
sweeps are employed to produce large cores that are not bodies of revolution. Various
machines may also be employed in the core-making process. These include die ex-
truders, jolt-squeeze machines, sandslingers, and pneumatic core blowers. Large cores
are handled in the foundry and placed into the mold by means of a crane.

Gating systems. Molds are filled with molten metal by means of channels, called
gates, cut in the sand of the mold. Figure 3.4 illustrates a typical gating system, which
includes a pouring basin, a down sprue, a sprue base(well), a runner, and in-gates. The
design of the gating system is sometimes critical and should, therefore, be based on the
theories of fluid mechanics, as well as the recommended industrial practice. In fact, a
gating system must be designed so that the following are ensured:

1. A continuous, uniform flow of molten metal into the mold cavity must be pro-
vided without any turbulence.

FIGURE 3.4

A typical gating system

Down sprue

n-gates

Sprue base (well)

Runner

40 3 Casting and Foundry Work

2. A reservoir of molten metal that feeds the casting to compensate for the shrinkage
during solidification must be maintained.

3. The molten metal stream must be prevented from separating from the wall of the
sprue.

Let us now break down the gating system into its components and discuss the de-
sign of each of them. The pouring basin is designed to reduce turbulence. The molten
metal from the ladle must be poured into the basin at the side that does not have the
tapered sprue hole. The hole should have a projection with a generous radius around
it, as shown in Figure 3.4, in order to eliminate turbulence as the molten metal enters
the sprue. Next, the down sprue should be made tapered (its cross-sectional area
should decrease when going downward) to prevent the stream of molten metal from
separating from its walls, which may occur because the stream gains velocity as it trav-
els downward and, therefore, contracts (remember the continuity equation in fluid me-
chanics, Ax V, = A2V2). The important and critical element of the gating system is the
in-gate, whose dimensions affect those of all other elements. Sometimes, the cross-
sectional area of the in-gate is reduced in the zone adjacent to the sprue base to create
a "choke area" that is used mainly to control the flow of molten metal and, conse-
quently, the pouring time. In other words, it serves to ensure that the rate of molten-
metal flow into the mold cavity is not higher than that delivered by the ladle and,
therefore, keeps the gating system full of metal throughout the casting operation.
On the other hand, gas contamination, slag inclusions, and the like should be elim-
inated by maintaining laminar flow. Accordingly, the Reynolds number (R„) should
be checked throughout the gating system (remember that the flow is laminar when
/?„ < 2000). Use must also be made of Bernoulli's equation to calculate the velocity of
flow at any cross section of the gating system.

In some cases, when casting heavy sections or high-shrinkage alloys, extra reser-
voirs of molten metal are needed to compensate continually for the shrinkage of the
casting during solidification. These molten-metal reservoirs are called risers and are
attached to the casting at appropriate locations to control the solidification process. The
locations of the feeding system and the risers should be determined based on the phe-
nomenon that sections most distant from those molten-metal reservoirs solidify first.
Risers are molded into the cope half of the mold to ensure gravity feeding of the
molten metal and are usually open to the top surface of the mold. In that case, they are
referred to as open risers. When they are not open to the top of the mold, they are then
called blind risers. Risers can also be classified as top risers and side risers, depend-
ing upon their location with respect to the casting.

Another way to achieve directional solidification is the use of chills; these involve
inserts of steel, cast iron, or copper that act as a "heat sink" to increase the solidifica-
tion rate of the metal at appropriate regions of the casting. Depending upon the shape
of the casting, chills can be external or internal.

Molding processes. Green sand can be molded by employing a variety of processes,
including some that are carried out both by hand and with molding machines. Follow-
ing is a brief survey of the different green sand molding methods:

3.1 Classifications of Casting by Mold Material

Flask molding. Flask molding is the most widely used process in both hand- and
machine-molding practices. Figure 3.5 illustrates the procedure for simple hand-
molding using a single (loose) pattern. First, the lower half of the pattern is placed
on a molding board and surrounded by the drag. The drag is then filled with sand
(using a shovel) and rammed very firmly. Ventilation holes are made using a steel
wire, but these should not reach the pattern. The drag is turned upside down to
bring the parting plane up so that it can be dusted. Next, the other half of the pat-
tern is placed in position to match the lower half, and the cope is located around it,
with the eyes of the cope fitted to the pins of the drag. Sand is shoveled into the
cope and rammed firmly, after using a sprue pin to provide for the feeding passage.
Ventilation holes are made in the cope part of the mold in the same way they were
made in the other half. The pouring basin is cut around the head of the sprue pin
using a trowel, and the sprue pin is pulled out of the cope. The cope is then care-
fully lifted off the drag and turned so that the parting plane is upward. The two
halves of the pattern are removed from both the cope and the drag. The runner
and/or gate are cut from the mold cavity to the sprue in the drag part of the mold.
Then, any damages are repaired by slightly wetting the location and using a slick.
The cope is then carefully placed on the drag to assemble the two halves of the

FIGURE 3.5

The procedure of flask
molding using a single
(loose) pattern

(1)

(2)

?zzzx;

(3)

(4)

42 3 Casting and Foundry Work

mold. Finally, the cope and the drag are fastened together by means of shackles or
bolts to prevent the pressure created by the molten metal (after pouring) from sep-
arating them. Enough weight can be placed on the cope as an alternative to using
shackles or bolts. In fact, the pressure of the molten metal after casting can be given
by the following equation:

p = wx h (3.1)

where: p is the pressure

w is the specific weight of the molten metal
h is the height of the cope

The force that is trying to separate the two halves of the mold can, therefore, be given
by the following equation:

F = p x A (3.2)

where: F is the force

A is the cross-sectional area of the casting (including the runner, gates,
etc.) at the parting line

2. Stack molding. Stack molding is best suited for producing a large number of small,
light castings while using a limited amount of floor space in the foundry. As can be
seen in Figure 3.6a and b, there are two types of stack molding: upright and
stepped. In upright stack molding, 10 to 12 flask sections are stacked up. They all
have a common sprue that is employed in feeding all cavities. The drag cavity is al-
ways molded in the upper surface of the flask section, whereas the cope cavity is
molded in the lower surface. In stepped stack molding, each section has its own
sprue and is, therefore, offset from the one under it to provide for the pouring basin.
In this case, each mold is cast separately.

3. Sweep molding. Sweep molding is used to form the surfaces of the mold cavity
when a large-size casting must be produced without the time and expenses involved
in making a pattern. A sweep that can be rotated around an axis is used for produc-
ing a surface of revolution, contrary to a drawing sweep, which is pushed axially
while being guided by a frame to produce a surface having a constant section along
its length (see discussion of the extrusion process in Chapter 5).

4. Pit molding. Pit molding is usually employed for producing a single piece of a large
casting when it would be difficult to handle patterns of that size in flasks. Molding
is done in specially prepared pits in the ground of the foundry. The bottom of the
pit is often covered with a layer of coke that is 2 to 3 inches (50 to 75 mm) thick.
Then, a layer of sand is rammed onto the coke to act as a "bed" for the mold. Vent
pipes connect the coke layer to the ground surface. Molding is carried out as usual,
and molds are almost always dried before pouring the molten metal. This drying is
achieved by means of a portable mold drier. A cope that is also dried is then placed
on the pit, and a suitable weight or a group of weights are located on the cope to
prevent it from floating when the molten metal is poured.

3.1 Classifications of Casting by Mold Material

FIGURE 3.6

The two types of stack
molding: (a) upright;
(b) stepped

(b)

Molding machines. The employment of molding machines results in an increase in
the production rate, a marked increase in productivity, and a higher and more con-
sistent quality of molds. The function of these machines is to pack the sand onto the
pattern and draw the pattern out from the mold. There are several types of molding
machines, each with a different way of packing the sand to form the mold. The main
types include squeezers, jolt machines, and sandslingers. There are also some ma-
chines, such as jolt-squeeze machines, that employ a combination of the working

3 Casting and Foundry Work

principles of two of the main types. Following is a brief discussion of the three main
types of molding machines (see Figure 3.7):

1. Squeezers. Figure 3.7a illustrates the working principle of the squeezer type of
molding machine. The pattern plate is clamped on the machine table, and a flask is
put into position. A sand frame is placed on the flask, and both are then filled with

FIGURE 3.7

Molding machines:

(a) squeezer;

(b) jolt machine;

Squeeze
head

Flask

3.1 Classifications of Casting by Mold Material 45

sand from a hopper. Next, the machine table travels upward to squeeze the sand be-
tween the pattern plate and a stationary head. The squeeze head enters into the sand
frame and compacts the sand so that it is level with the edge of the flask.

2. Jolt machines. Figure 3.7b illustrates the working principle of the jolt type of
molding machine. As can be seen, compressed air is admitted through the hose to a
pressure cylinder to lift the plunger (and the flask, which is full of sand) up to a cer-
tain height, where the side hole is uncovered to exhaust the compressed air. The
plunger then falls down and strikes the stationary guiding cylinder. The shock wave
resulting from each of the successive impacts contributes to packing the molding
sand in the flask.

3. Sandslingers. Figure 3.7c shows a sandslinger. This type of machine is employed
in molding sand in flasks of any size, whether for individual or mass production of
molds. Sandslingers are characterized by their high output, which amounts to 2500
cubic feet (more than 60 cubic meters) per hour. As can be seen, molding sand is
fed into a housing containing an impeller that rotates rapidly around a horizontal
axis. Sand particles are picked up by the rotating blades and thrown at a high speed
through an opening onto the pattern, which is located in the flask.

No matter what type of molding machine is used, special machines are employed
to draw the pattern out of the mold. Basically, these machines achieve that goal by
turning the flask (together with the pattern) upside down and then lifting the pattern
out of the mold. Examples of these machines include roll-over molding machines and
rock-over pattern-draw machines.

Sand conditioning. The molding sand, whether new or used, must be conditioned be-
fore being used. When used sand is to be recycled, lumps should be crushed and then
metal granules or small parts removed (a magnetic field is employed in a ferrous
foundry). Next, sand (new or recycled) and all other molding constituents must be
screened in shakers, rotary screens, or vibrating screens. Molding materials are then
thoroughly mixed in order to obtain a completely homogeneous green sand mixture.
The more uniform the distribution, the better the molding properties (like permeability
and green strength) of the sand mixture will be.

Mixing is carried out in either continuous-screw mixers or vertical-wheel mullers.
The mixers mix the molding materials by means of two large screws or worm gears;
the mullers are usually used for batch-type mixing. A typical muller is illustrated in
Figure 3.8. It consists primarily of a pan in which two wheels rotate about their own
horizontal axis as well as about a stationary vertical shaft. Centrifugal mullers are also
in use, especially for high production rates.

Dry Sand Molds

As previously mentioned, green sand molds contain up to 8 percent water, depending
upon the kind and percentage of the binding material. Therefore, this type of mold can
be used only for small castings with thin walls; large castings with thick walls would
heat the mold, resulting in vaporization of water, which would, in turn, lead to bubbles

3 Casting and Foundry Work

FIGURE 3.8

A muller for sand
conditioning

Plow blade

Outlet

(conditioned sand)

Wheels

in the castings. For this reason, molds for large castings should be dried after they
are made in the same way as green sand molds. The drying operation is carried out
in ovens at temperatures ranging from 300°F to 650°F (150°C to 350°C) for 8 up to
48 hours, depending upon the kind and amount of binder used.

Core-Sand Molds

When the mold is too big to fit in an oven, molds are made by assembling several
pieces of sand cores. Consequently, patterns are not required, and core boxes are em-
ployed instead to make the different sand cores necessary for constructing the mold.
Because core-sand mixtures (which have superior molding properties) are used, very
good quality and dimensional accuracy of the castings are obtained.

Cement-Bonded Sand Molds

A mixture of silica sand containing 8 to 12 percent cement and 4 to 6 percent water is
used. When making the mold, the cement-bonded sand mixture must be allowed to
harden first before the pattern is withdrawn. The obtained mold is then allowed to cure
for about 3 to 5 days. Large castings with intricate shapes, accurate dimensions, and
smooth surfaces are usually produced in this way, the only shortcoming being the long
time required for the molding process.

Carbon Dioxide Process for Molding

Silica sand is mixed with a binder involving a solution of sodium silicate (water glass)
amounting to 6 percent. After the mold is rammed, carbon dioxide is blown through
the sand mixture. As a result, the gel of silica binds the sand grains together, and no

3.1 Classifications of Casting by Mold Material 47

drying is needed. Because the molds are allowed to harden while the pattern is in po-
sition, high dimensional accuracy of molds is obtained.

Plaster Molds

A plaster mold is appropriate for casting silver, gold, magnesium, copper, and alu-
minum alloys. The molding material is a mixture of fine silica sand, asbestos, and plas-
ter of paris as a binder. Water is added to the mixture until a creamy slurry is obtained,
which is then employed in molding. The drying process should be very slow to avoid
cracking of the mold.

Loam Molds

The loam mold is used for very large jobs. The basic shape of the desired mold is con-
structed with bricks and mortar (just like a brick house). A loam mixture is then used
as a molding material to obtain the desired fine details of mold. Templates, sweeps,
and the like are employed in the molding process. The loam mixture used in molding
consists of 50 percent or more of loam, with the rest being mainly silica sand. Loam
molds must be thoroughly dried before pouring the molten metal.

Shell Molds

In shell molding, a thin mold is made around a heated-metal pattern plate. The mold-
ing material is a mixture of dry, fine silica sand (with a very low clay content) and 3
to 8 percent of a thermosetting resin like phenolformaldehyde or ureaformaldehyde.
Conventional dry-mixing techniques are used for obtaining the molding mixture. Spe-
cially prepared resin-coated sands are also used.

When the molding mixture drops onto the pattern plate, which is heated to a tem-
perature of 350°F to 700°F (180°C to 375°C), a shell about 1/4 inch (6 mm) thick is
formed. In order to cure the shell completely, it must be heated at 450°F to 650°F
(230°C to 350°C) for about 1 to 3 minutes. The shell is then released from the pattern
plate by ejector pins. To prevent sticking of the baked shell, sometimes called the bis-
cuit, to the pattern plate, a silicone release agent is applied to the plate before the mold-
ing mixture drops onto it. Figure 3.9 is a photograph of a pattern of a crankshaft used
in shell molding.

Shell molding is suitable for mass production of thin-walled, gray cast-iron (and
aluminum-alloy) castings having a maximum weight between 35 and 45 pounds (15
and 20 kg). However, castings weighing up to 1000 pounds (450 kg) can be made by
employing shell molding on an individual basis. The advantages of shell molding in-
clude good surface finish, few restrictions on casting design, and the fact that this
process renders itself suitable for automation.

Ceramic Molds

In the ceramic molding process, the molding material is actually a slurry consisting
of refractory grains, ceramic binder, water, alcohol, and an agent to adjust the pH
value (see discussion of slurry casting in Chapter 7). The slurry is poured around the

3 Casting and Foundry Work

FIGURE 3.9

A pattern of a
crankshaft used in shell
molding

m\ If rlt-LJ I II W I ^H

permanent (reusable) pattern and is allowed to harden when the pattern is withdrawn.
Next, the mold is left to dry for some time and then is fired to gain strength. In fact,
ceramic molds are usually preheated before pouring the molten metal. For this rea-
son, they are suitable for casting high-pouring-temperature alloys. Excellent surface
finish and very close tolerances of the castings are among the advantages of this
molding process and lead to the elimination of the machining operations that are usu-
ally performed on castings. Therefore, ceramic molds are certainly advantageous
when casting precious or difficult-to-machine metals as well as for making castings
with great shape intricacy.

Precision Molds (Investment Casting)

Precision molding is used when castings with intricate shapes, good dimensional ac-
curacy, and very smooth surfaces are required. The process is especially advantageous
for high-melting-point alloys as well as for difficult-to-machine metals. It is also most
suitable for producing small castings having intricate shapes, such as the group of in-
vestment castings shown in Figure 3.10. A nonpermanent pattern that is usually made
of wax must be prepared for each casting. Therefore, the process is sometimes referred
to as the lost-wax process. Generally, the precision molding process involves the fol-
lowing steps (see Figure 3.11):

1. A heat-disposable pattern, together with its gating system, is prepared by injecting
wax or plastic into a die cavity.

2. A pattern assembly that is composed of a number of identical patterns is made. Pat-
terns are attached to a runner bar made of wax or plastic in much the same manner
as leaves are attached to branches. A ceramic pouring cup is also attached to the top
of the pattern assembly, which is sometimes referred to as the tree or cluster (see
Figure 3.11a).

3.1 Classifications of Casting by Mold Material

FIGURE 3.10

A group of investment
castings (Courtesy of
Fansteel ESCAST,
Addison, Illinois)

'^*M>\

\ \

3. The tree is then invested by separately dipping it into a ceramic slurry that is com-
posed of silica flour suspended in a solution of ethyl silicate and sprinkling it with
very fine silica sand. A self-supporting ceramic shell mold about 1/4 inch (6 mm)
thick is formed all around the wax assembly (see Figure 3.1 lb). Alternatively, a thin
ceramic precoating is obtained, and then the cluster is placed in a flask and a thick
slurry is poured around it as a backup material.

4. The pattern assembly is then baked in an oven or a steam autoclave to melt out the
wax (or plastic). Therefore, the dimensions of the mold cavity precisely match
those of the desired product.

5. The resulting shell mold is fired at a temperature ranging from 1600°F to 1800°F
(900°C to 1000°C) to eliminate all traces of wax and to gain reasonable strength.

6. The molten metal is poured into the mold while the mold is still hot, and a cluster
of castings is obtained (see Figure 3.11c).

Today, the lost-wax process is used in manufacturing large objects like cylinder
heads and camshafts. The modern process, which is known as the lost-foam method,
involves employing a styrofoam replica of the finished product, which is then coated
with a refractory material and located in a box, where sand is molded around it by vi-
bratory compaction. When the molten metal is finally poured into the mold, the styro-
foam vaporizes, allowing the molten metal to replace it.

3 Casting and Foundry Work

FIGURE 3.11

Steps involved in investment casting: (a) a cluster of wax patterns; (b) a cluster of ceramic shells;
(c) a cluster of castings (Courtesy of Fansteel ESCAST, Addison, Illinois)

Graphite Molds

Graphite is used in making molds to receive alloys (such as titanium) that can be
poured only into inert molds. The casting process must be performed in a vacuum to
eliminate any possibility of contaminating the metal. Graphite molds can be made ei-
ther by machining a block of graphite to create the desired mold cavity or by com-
pacting a graphite-base aggregate around the pattern and then sintering the obtained
mold at a temperature of 18()0°F to 2000°F (1000°C to 1120°C) in a reducing atmos-
phere (see Chapter 7). In fact, graphite mold liners have found widespread industrial
application in the centrifugal casting of brass and bronze.

3.1 Classifications of Casting by Mold Material

Permanent Molds

A permanent mold can be used repeatedly for producing castings of the same form and
dimensions. Permanent molds are usually made of steel or gray cast iron. Figure 3.12a
and b shows a permanent mold made of alloy steel for molding a cylinder block. Each
mold is generally made of two or more pieces that are assembled together by fitting
and clamping. Although the different parts of the mold can be cast to their rough con-
tours, subsequent machining and finishing operations are necessary to eliminate the
possibility of the casting's sticking to the mold. Simple cores made of metal are fre-
quently used. When complex cores are required, they are usually made of sand or plas-
ter, and the mold is said to be semipermanent.

Different metals and alloys can successfully be cast in permanent molds. They in-
clude aluminum alloys, magnesium alloys, zinc alloys, lead, copper alloys, and cast

FIGURE 3.12

A permanent mold
made of alloy steel for
casting a cylinder block:
(a) drag; (b) cope

52 3 Casting and Foundry Work

irons. It is obvious that the mold should be preheated to an appropriate temperature
prior to casting. In fact, the operating temperature of the mold, which depends upon
the metal to be cast, is a very important factor in successful permanent-mold casting.

Based on the preceding discussion, we can expect the mold life to be dependent
upon a number of interrelated factors, including the mold material, the metal to be cast,
and the operating temperature of the mold. Nevertheless, it can be stated that the life
of a permanent mold is about 100,000 pourings or more when casting zinc, magne-
sium, or aluminum alloys and not more than 20,000 pourings for copper alloys and
cast irons. However, mold life can be extended by spraying the surface of the mold
cavity with colloidal refractories suspended in liquids.

The advantages of permanent-mold casting include substantial increases in pro-
ductivity (a mold does not have to be made for each casting), close tolerances, supe-
rior surface finish, and improved mechanical properties of the castings. A further
advantage is the noticeable reduction in the percentage of rejects when compared with
the conventional sand-casting processes. Nevertheless, the process is economically
feasible for mass production only. There is also a limitation on the size of parts pro-
duced by permanent-mold casting. A further limitation is that not all alloys are suited
to this process.

3.2 CLASSIFICATIONS OF CASTING

BY METHOD OF FILLING THE MOLD

For all types of molds that we have discussed, the molten metal is almost always fed
into the mold only by the action of gravity. Therefore, the casting process is referred
to as gravity casting. There are, however, other special ways of pouring or feeding the
molten metal into the desired cavities. These casting methods are generally aimed at
forcing the molten metal to flow and fill the fine details of the mold cavity while elim-
inating the internal defects experienced in conventional gravity casting processes. Fol-
lowing is a survey of the commonly used special casting processes.

Die Casting

Die casting involves forcing the molten metal into the cavity of a steel mold, called a
die, under very high pressure (1000 to 30,000 pounds per square inch, or about 70 to
2000 times the atmospheric pressure). In fact, this characteristic is the major difference
between die casting and permanent-mold casting, where the molten metal is fed into
the mold either by gravity or at low pressures. Die casting may be classified according
to the type of machine used. The two principal types are hot-chamber machines and
cold-chamber machines.

Hot-chamber machines. The main components of the hot-chamber die casting ma-
chine include a steel pot filled with the molten metal to be cast and a pumping system
that consists of a pressure cylinder, a plunger, a gooseneck passage, and a nozzle. With
the plunger in the up position, as shown in Figure 3.13a, the molten metal flows by
gravity through the intake ports into the submerged hot chamber. When the plunger is

3.2 Classifications of Casting by Method of Filling the Mold

FIGURE 3.13

The hot-chamber die casting method: (a) filling the chamber; (b) metal forced into the die cavity

Plunger

Hot pot

Inlets

Burner

Movable

die ^f Stationary

die half

(a)

(b)

pushed downward by the power cylinder (not shown in the figure), it shuts off the in-
take port. Then, with further downward movement, the molten metal is forced through
the gooseneck passage and the nozzle into the die cavity, as shown in Figure 3.13b.
Pressures ranging from 700 to 2000 pounds per square inch (50 to 150 atmospheres)
are quite common to guarantee complete filling of the die cavity. After the cavity is full
of molten metal, the pressure is maintained for a preset dwell time to allow the casting
to solidify completely. Next, the two halves of the die are pushed apart, and the cast-
ing is knocked out by means of ejector pins. The die cavity is then cleaned and lubri-
cated before the cycle is repeated.

The advantages of hot-chamber die casting are numerous. They include high pro-
duction rates (especially when multicavity dies are used), improved productivity, su-
perior surface finish, very close tolerances, and the ability to produce intricate shapes
with thin walls. Nevertheless, the process has some limitations. For instance, only low-
melting-point alloys (such as zinc, tin, lead, and the like) can be cast because the com-
ponents of the pumping system are in direct contact with the molten metal throughout
the process. Also, die casting is usually only suitable for producing small castings that
weigh less than 10 pounds (4.5 kg).

Cold-chamber machines. In the cold-chamber die casting machine, the molten-metal
reservoir is separate from the casting machine, and just enough for one shot of
molten metal is ladled every stroke. Consequently, the relatively short exposure of
the shot chamber and the plunger to the molten metal allows die casting of alu-
minum, magnesium, brass, and other alloys having relatively high melting points. In
the sequence of operations in cold-chamber die casting, the molten metal is first la-
dled through the pouring hole of the shot chamber while the two halves of the die are
closed and locked together, as shown in Figure 3.14. Next, the plunger moves for-
ward to close off the pouring hole and then forces the molten metal into the die cav-
ity. Pressures in the shot chamber may go over 30,000 pounds per square inch (2000

3 Casting and Foundry Work

FIGURE 3.14

The cold-chamber die
casting method

Ejector

Ladle

Plunger

atmospheres). After the casting has solidified, the two halves of the die are opened,
and the casting, together with the gate and the slug of excess metal, are ejected from
the die.

It is not difficult to see that large parts weighing 50 pounds (23 kg) can be pro-
duced by cold-chamber die casting. The process is very successful when casting alu-
minum alloys, copper alloys, and high-temperature aluminum-zinc alloys. However,
this process has a longer cycle time when compared with hot-chamber die casting. A
further disadvantage is the need for an auxiliary system for pouring the molten metal.
It is mainly for this reason that vertical cold-chamber machines were developed. As
can be seen in Figure 3.15, such a machine has a transfer tube that is submerged into
molten metal. It is fed into the shot chamber by connecting the die cavity to a vacuum
tank by means of a special valve. The molten metal is forced into the die cavity when
the plunger moves upward.

Centrifugal Casting

Centrifugal casting refers to a group of processes in which the forces used to distrib-
ute the molten metal in the mold cavity (or cavities) are caused by centrifugal acceler-
ation. Centrifugal casting processes can be classified as true centrifugal casting,

FIGURE 3.15

A vertical cold-chamber
die casting machine

Vacuum

3.2 Classifications of Casting by Method of Filling the Mold

semicentrifugal casting, and the centrifuging method. Each of these processes is briefly
discussed next.

True centrifugal casting. True centrifugal casting involves rotating a cylindrical mold
around its own axis, with the revolutions per minute high enough to create an effective
centrifugal force, and then pouring molten metal into the mold cavity. The molten metal
is pushed to the walls of the mold by centrifugal acceleration (usually 70 to 80 times that
of gravity), where it solidifies in the form of a hollow cylinder. The outer shape of the
casting is given by the mold contour, while the diameter of the inner cylindrical surface
is controlled by the amount of molten metal poured into the mold cavity. The machines
used to spin the mold may have either horizontal or vertical axes of rotation. Short tubes
are usually cast in vertical-axis machines, whereas longer pipes, like water supply and
sewer pipes, are cast using horizontal-axis machines. The basic features of a true cen-
trifugal casting machine with a horizontal axis are shown in Figure 3.16.

Centrifugal castings are characterized by their high density, refined fine-grained
structure, and superior mechanical properties, accompanied by a low percentage of re-
jects and, therefore, a high production output. A further advantage of the centrifugal
casting process is the high efficiency of metal utilization due to the elimination of
sprues and risers and the small machining allowance used.

Semicentrifugal casting. Semicentrifugal casting is quite similar to the preceding
type, the difference being that the mold cavity is completely filled with the molten
metal. But because centrifugal acceleration is dependent upon the radius, the central
core of the casting is subjected to low pressure and is, therefore, the region where en-
trapped air and inclusions are present. For this reason, the semicentrifugal casting
process is recommended for producing castings that are to be subjected to subsequent
machining to remove their central cores. Examples include cast track wheels for tanks,

FIGURE 3.16

A true centrifugal
casting machine

Motor

Spacers

Ladle

56 3 Casting and Foundry Work

tractors, and the like. A sand core is sometimes used to form the central cavity of the
casting in order to eliminate the need for subsequent machining operations.

Centrifuging. In the centrifuging method, a number of mold cavities are arranged on
the circumference of a circle and are connected to a central down sprue through radial
gates. Next, molten metal is poured, and the mold is rotated around the central axis of
the sprue. In other words, each casting is rotated around an axis off (shifted from) its
own center axis. Therefore, mold cavities are filled under high pressure, so the process
is usually used for producing castings with intricate shapes; the increased pressure on
the casting during solidification allows the fine details of the mold to be obtained.

Continuous Casting

The continuous casting process is gaining widespread industrial use, especially for
high-quality alloy steel. In fact, the process itself passed through a few evolutionary
stages. Although it was originally developed for producing cast-iron sheets, an up-to-
date version is now being used for casting semifinished products that are to be
processed subsequently by piercing, forging, extrusion, and the like.

The continuous casting process basically involves controlling the flow of a stream
of molten metal that comes out from a water-cooled orifice in order to solidify and
form a continuous strip (or rod). The new version of this process is usually referred to
as rotary continuous casting because the water-cooled mold (orifice) is always oscil-
lating and rotating at about 120 revolutions per minute during casting. Figure 3.17 il-
lustrates the principles of rotary continuous casting. The steel is melted, refined, and
degassed and its chemical composition controlled before it is transferred and poured
into the caster (tundish). The molten metal then enters the rotating mold tangent to the
edge through the bent tube. The centrifugal force then forces the steel against the mold
wall, while lighter inclusions and impurities remain in the center of the vortex, where
they are removed by the operator. Solidification of the metal flowing out of the mold
continues at a precalculated rate. The resulting bar is then cut by a circular saw that is
traveling downward at the same speed as the bar. The bar is tilted and loaded onto a
conveyor to transfer it to the cooling bed and the rolling mill.

The continuous casting process has the advantages of very high metal yield
(about 98 percent, compared with 87 percent in conventional ingot-mold practice),
excellent quality of cast, controlled grain size, and the possibility of casting special
cross-sectional shapes.

The V-Process

The vacuum casting process (V-process for short) involves covering the two halves of
the pattern with two plastic films that are 0.005 inch (0.125 mm) thick by employing
vacuum forming (see chapter 8). The pattern is then removed, and the two formed-
plastic sheets are tightened together to form a mold cavity that is surrounded by a flask
filled with sand (there is no need for a binder). This mold cavity is kept in a vacuum
as the molten metal is poured to assist and ensure easy flow.

3.2 Classifications of Casting by Method of Filling the Mold

FIGURE 3.17

The principles of rotary
continuous casting

Bent tube

0
0

O
O

O
0

Ladle

Ceramic tube

Tundish

Casting

Guiding rolls

Extractor

Hot saw (it travels

downwards

while cutting)

Product

Conveyor to rolling mill

The V-process, developed in Japan in the early 1970s, offers many advantages,
such as the elimination of the need for special molding sands with binders and the
elimination of the problems associated with green sand molding (like gas bubbles
caused by excess humidity). Also, the size of risers, vents, and sprues can be reduced
markedly, thus resulting in an increase in the efficiency of material utilization. Figure
3.18 shows a plastic mold being prepared for the V-process.

3 Casting and Foundry Work

FIGURE 3.18

A plastic mold being prepared for the V-process (Courtesy of Spectrum Casting, Inc., Flint,
Michigan)

3.3 CLASSIFICATIONS OF CASTING
BY METAL TO BE CAST

When classified by metal, castings can be either ferrous or nonferrous. The ferrous
castings include cast steels and the family of cast irons, whereas the nonferrous cast-
ings include all other metals, such as aluminum, copper, magnesium, titanium, and
their alloys. Each of these metals and alloys is melted in a particular type of foundry
furnace that may not be appropriate for melting other metals and alloys. Also, molding
methods and materials, as well as fluxes, degassers, and additives, depend upon the
metal to be cast. Therefore, this classification method is popular in foundry work. Fol-
lowing is a brief discussion of each of these cast alloys.

Ferrous Metals

Cast steels. Steels are smelted in open-hearth furnaces, convertors, electric-arc fur-
naces, and electric-induction furnaces. Cast steels can be either plain-carbon, low-
alloy, or high-alloy steel. However, plain-carbon cast steel is the most commonly
produced type. When compared with cast iron, steel certainly has poorer casting prop-
erties— namely, higher melting point, higher shrinkage, and poorer fluidity. Steels are
also more susceptible to hot and cold cracks after the casting process. Therefore, cast

3.3 Classifications of Casting by Metal to Be Cast 59

steels are almost always subjected to heat treatment to relieve the internal stresses and
improve the mechanical properties.

In order to control the oxygen content of molten steels, aluminum, silicon, or man-
ganese is used as a deoxidizer. Aluminum is the most commonly used of these ele-
ments because of its availability, low cost, and effectiveness.

There is an important difference between cast-steel and wrought products. This in-
volves the presence of a "skin," or thin layer, just below the surface of a casting, where
scales, oxides, and impurities are concentrated. Also, this layer may be chemically or
structurally different from the base metal. Therefore, it has to be removed by machin-
ing in a single deep cut, which is achieved through reducing the cutting speed to half
of the conventionally recommended value.

Gray cast iron. Gray cast iron is characterized by the presence of free graphite flakes
when its microstructure is examined under the microscope. This kind of microstructure
is, in fact, responsible for the superior properties possessed by gray cast iron. For in-
stance, this dispersion of graphite flakes acts as a lubricant during machining of gray
cast iron, thus eliminating the need for machining lubricants and coolants. When com-
pared with any other ferrous cast alloy, gray cast iron certainly possesses superior
machinability. The presence of those graphite flakes is also the reason for its ability to
absorb vibrations. The compressive strength of this iron is normally four times its ten-
sile strength. Thus, gray cast iron has found widespread application in machine tool
beds (bases) and the like. On the other hand, gray cast iron has some disadvantages
and limitations, such as its low tensile strength, brittleness, and poor weldability. Nev-
ertheless, gray cast iron has the lowest casting temperature, least shrinkage, and the
best castability of all cast ferrous alloys.

The cupola is the most widely used foundry furnace for producing and melting
gray cast iron. The chemical composition, microstructure, and, therefore, the proper-
ties of the obtained castings are determined by the constituents of the charge of the
cupola furnace. Thus, the composition and properties of gray cast iron are controlled
by changing the percentages of the charge constituents and also by adding inoculants
and alloying elements. Commonly used inoculants include calcium silicide, ferrosili-
con, and ferromanganese. An inoculant is added to the molten metal (either in the
cupola spout or ladle) and usually amounts to between 0.1 and 0.5 percent of the
molten iron by weight. It acts as a deoxidizer and also hinders the growth of precipi-
tated graphite flakes. It is important for a product designer to remember that the prop-
erties of a gray cast-iron product are also dependent upon the dimensions (the
thicknesses of the walls) of that product because the cooling rate is adversely affected
by the cross section of the casting. Actually, the cooling rate is high for small castings
with thin walls, sometimes yielding white cast iron. For this reason, gray cast iron
must be specified by the strength of critical cross sections.

White cast iron. When the molten cast-iron alloy is rapidly chilled after being
poured into the mold cavity, dissolved carbon does not have enough time to precipi-
tate in the form of flakes. Instead, it remains chemically combined with iron in the
form of cementite. This material is primarily responsible for the whitish crystalline
appearance of a fractured surface of white cast iron. Cementite is also responsible for

60 3 Casting and Foundry Work

the high hardness, extreme brittleness, and excellent wear resistance of this kind of
cast iron. Industrial applications of white cast iron involve components subjected to
abrasion. Sometimes, gray cast iron can be chilled to produce a surface layer of white
cast iron in order to combine the advantageous properties of the two types of cast
iron. In this case, the product metal is usually referred to as chilled cast iron.

Ductile cast iron. Ductile cast iron is also called nodular cast iron and spheroidal-
graphite cast iron. It is obtained by adding trace amounts of magnesium to a very
pure molten alloy of gray cast iron that has been subjected to desulfurization. Some-
times, a small quantity of cerium is also added to prevent the harmful effects of im-
purities like aluminum, titanium, and lead. The presence of magnesium and cerium
causes the graphite to precipitate during solidification of the molten alloy in the form
of small spheroids, rather than flakes as in the case of gray cast iron. This mi-
crostructural change results in a marked increase in ductility, strength, toughness, and
stiffness of ductile iron, as compared with gray cast iron, because the stress concen-
tration effect of a flake is far higher than that of a spheroid (remember what you
learned in fracture mechanics). The disadvantages of ductile iron, as compared with
gray cast iron, include lower damping capacity and thermal conductivity. Ductile iron
is used for making machine parts like axles, brackets, levers, crankshafts, housings,
die pads, and die shoes.

Compacted-graphite cast iron. Compacted-graphite (CG) cast iron falls between gray
and ductile cast irons, both in its microstructure and mechanical properties. The free
graphite in this type of iron takes the form of short, blunt, and interconnected flakes.
The mechanical properties of CG cast iron are superior to those of gray cast iron but
are inferior to those of ductile cast iron. The thermal conductivity and damping capac-
ity of CG cast iron approach those of gray cast iron. Compacted-graphite cast iron has
some application in the manufacture of diesel engines.

Malleable cast iron. Malleable cast iron is obtained by two-stage heat treatment of
white cast iron having an appropriate chemical composition. The hard white cast
iron becomes malleable after the heat treatment due to microstructural changes. The
combined carbon separates as free graphite, which takes the form of nodules. Be-
cause the raw material for producing malleable iron is actually white cast iron, there
are always limitations on casting design. Large cross sections and thick walls are
not permitted because it is difficult to produce a white cast-iron part with these
geometric characteristics.

The two basic types of malleable cast iron are the pearlitic and the ferritic (black-
heart). Although the starting alloy for both types is the same (white cast iron), the heat
treatment cycle and the atmosphere of the heat-treating furnace are different in each
case. Furnaces with oxidizing atmospheres are employed for producing pearlitic mal-
leable cast iron, whereas furnaces with neutral atmospheres are used for producing fer-
ritic malleable cast iron. When comparing the properties of these two types, the ferritic
grades normally have higher ductility and better machinability but lower strength and
hardness. Pearlitic grades can, however, be subjected to further surface hardening
when the depth of the hardened layer is controlled.

3.3 Classifications of Casting by Metal to Be Cast

FIGURE 3.19

The heat treatment
sequence for producing
malleable cast iron

Temperature ,,
(°Cor°F

(850-950°C) 1700°F
(800°C) 1400°F

First stage

per hour

"/-■
I

-X^ cior-nnH stage 5°C

Time (hours)

6 hours

-* Up to 100 hours +~

Figure 3.19 shows the heat treatment sequence for producing malleable cast iron.
Referred to as the malleabilizing cycle, it includes two stages, as shown in Figure 3.19.
In the first stage, the casting is slowly heated to a temperature of about 1700°F (950°C)
and is kept at that temperature for about 24 hours. In the second stage, the temperature
is decreased very slowly at a rate of 5°F to 9°F (3°C to 5°C) per hour from a temper-
ature of 1400°F (800°C) to a temperature of 1200°F (650°C), where the process ends
and the casting is taken out of the furnace. The whole malleabilizing cycle normally
takes about 100 hours.

Malleable cast iron is usually selected when the engineering application requires
good machinability and ductility. Excellent castability and high toughness are other
properties that make malleable cast iron attractive as an engineering material. Typical
applications of malleable cast iron include flanges, pipe fittings, and valve parts for
pressure service at elevated temperatures, steering-gear housings, mounting brackets,
and compressor crankshafts and hubs.

Alloyed cast irons. Alloying elements like chromium, nickel, and molybdenum are
added to cast irons to manipulate the microstructure of the alloy. The goal is to im-
prove the mechanical properties of the casting and also to impart some special proper-
ties to it, like resistance to wear, corrosion, and heat. A typical example of alloyed
irons is the white cast iron containing nickel and chromium that is used for corrosion-
resistant (and abrasion-resistant) applications like water pump housings and grinding
balls (in a ball mill).

Nonferrous Metals

Cast aluminum and its alloys. Aluminum continues to gain wide industrial applica-
tion, especially in the automotive and electronics industries, because of its distin-
guished strength-to-weight ratio and its high electrical conductivity. Alloying elements
can be added to aluminum to improve its mechanical properties and metallurgical
characteristics. Silicon, magnesium, zinc, tin, and copper are the elements most com-
monly alloyed with aluminum. In fact, most metallic elements can be alloyed with

62 3 Casting and Foundry Work

aluminum, but commercial and industrial applications are limited to those just men-
tioned.

A real advantage of aluminum is that it can be cast by almost all casting processes.
Nevertheless, the common methods for casting aluminum include die casting, gravity
casting in sand and permanent molds, and investment casting (the lost-foam process).

The presence of hydrogen when melting aluminum always results in unsound
castings. Typical sources of hydrogen are the furnace atmosphere and the charge metal.
When the furnace has a reducing atmosphere because of incomplete combustion of the
fuel, carbon monoxide and hydrogen are generated and absorbed by the molten metal.
The presence of contaminants like moisture, oil, or grease, which are not chemically
stable at elevated temperatures, can also liberate hydrogen. Unfortunately, hydrogen is
highly soluble in molten aluminum but has limited solubility in solidified aluminum.
Therefore, any hydrogen that is absorbed by the molten metal is liberated or expelled
during solidification, causing porosity. Hydrogen may also react with (and reduce)
metallic oxides to form water vapor, which again causes porosity. Thus, hydrogen must
be completely removed from molten aluminum before casting. This is achieved by
using appropriate degassers. Chlorine and nitrogen are considered to be the traditional
degassers for aluminum. Either of these is blown through the molten aluminum to
eliminate any hydrogen. However, because chlorine is toxic and nitrogen is not that ef-
ficient, organic chloride fluxing compounds (chlorinated hydrocarbons) are added to
generate chlorine within the melt. They are commercially available in different forms,
such as blocks, powders, and tablets; the most commonly used fluxing degasser is per-
haps hexachlorethane. Another source of problems when casting aluminum is iron,
which dissolves readily in molten aluminum. Therefore, care must be taken to spray
(or cover) iron ladles and all iron surfaces that come into direct contact with the molten
aluminum with a ceramic coating. This extends the service life of the iron tools used
and also results in sound castings.

The most important cast-aluminum alloys are those containing silicon, which
serves to improve the castability, reduce the thermal expansion, and increase the wear
resistance of aluminum. Small additions of magnesium make these alloys heat treat-
able, thus allowing the final properties of the castings to be controlled. Aluminum-
silicon alloys (with 5 to 13 percent silicon) are used in making automobile parts (e.g.,
pistons) and aerospace components.

Aluminum-copper alloys are characterized by their very high tensile-strength-to-
weight ratio. They are, therefore, mainly used for the manufacture of premium-quality
aerospace parts. Nevertheless, these alloys have poorer castability than the aluminum-
silicon alloys. Also, amounts of the copper constituent in excess of 12 percent make
the alloy brittle. Copper additions of up to 5 percent are usually used and result in im-
proved high-temperature properties and machinability.

Additions of magnesium to aluminum result in improved corrosion resistance and
machinability, higher strength, and attractive appearance of the casting when anodized.
However, aluminum-magnesium alloys are generally difficult to cast. Zinc is also used
as an alloying element, and the aluminum-zinc alloys have good machinability and mod-
erately high strength. But these alloys are generally prone to hot cracking and have
poorer castability and high shrinkage. Therefore, zinc is usually alloyed with aluminum

3.4 Foundry Furnaces 63

in combination with other alloying elements and is employed in such cases for pro-
moting very high strength. Aluminum-tin alloys are also in use. They possess high load-
carrying capacity and fatigue strength and are, therefore, used for making bearings and
bushings.

Cast copper alloys. The melting temperatures of cast copper alloys are far higher
than those of aluminum, zinc, or magnesium alloys. Cast copper alloys can be grouped
according to their composition as follows:

1. Pure copper and high-copper alloys

2. Brasses (alloys including zinc as the principal alloying element)

3. Bronzes (alloys including tin as the principal alloying element)

4. Nickel silvers, including copper-nickel alloys and copper-nickel-zinc alloys

Cast copper alloys are melted in crucible furnaces, open-flame furnaces, induction
furnaces, or indirect-arc furnaces. The selection of a furnace depends upon the type of
alloy to be melted, as well as the purity and quantity required. In melting pure copper,
high-copper alloys, bronzes, or nickel silver, precautions must be taken to prevent con-
tamination of the molten metal with hydrogen. It is recommended that the atmosphere
of the furnace be slightly oxidizing and also that a covering flux be used. Prior to cast-
ing, however, the molten metal should be deoxidized by adding phosphorus in the form
of a phosphorous copper flux. On the other hand, brass is usually not susceptible to hy-
drogen porosity. The problem associated with melting brass is the vaporization and ox-
idation of the zinc. As a remedy, the atmosphere of the furnace should be slightly
reducing. Also, a covering flux should be used to prevent vaporization of the zinc; a
deoxidizing flux (like phosphorous copper) is then added immediately prior to pouring.
The applications of cast-copper alloys include pipe fitting, ornaments, propeller hubs
and blades, steam valves, and bearings.

Zinc alloys. The family of zinc alloys is characterized by low melting temperatures.
Zinc alloys also possess good fluidity. Therefore, they can be produced in thin sections
by submerged-hot-chamber die casting. Alloying elements employed include alu-
minum, copper, and magnesium.

Magnesium alloys. The main characteristic of magnesium is its low density, which is
lower than that of any other commercial metal. The potential uses of magnesium are
many because it is readily available as a component of seawater and most of its disad-
vantages and limitations can be eliminated by alloying. Magnesium alloys usually are
cast in permanent molds or are produced by hot-chamber die casting.

3.4 FOUNDRY FURNACES

Various furnaces are employed for smelting different ferrous and nonferrous metals in
foundry work. The type of foundry furnace to be used is determined by the kind of
metal to be melted, the hourly output of molten metal required, and the purity desired.
Following is a brief review of each of the commonly used foundry furnaces.

3 Casting and Foundry Work

Cupola Furnaces

Structure. The cupola is the most widely used furnace for producing molten gray
cast iron. A sketch of a cupola furnace is given in Figure 3.20. As can be seen, the
cupola is a shaft-type furnace whose height is three to five times its diameter. It is
constructed of a steel plate that is about 3/8 inch (10 mm) thick and that is internally
lined with refractory fireclay bricks. The whole structure is erected on legs, or
columns. Toward the top of the furnace is an opening through which the charge is
fed. Air, which is needed for the combustion, is blown through the tuyeres located
about 36 inches (900 mm) above the bottom of the furnace. Slightly above the bot-
tom and in the front are a tap hole and spout to allow molten cast iron to be col-
lected. There is also a slag hole located at the rear and above the level of the tap hole

FIGURE 3.20

A cupola furnace

Steel sheet

Refractory
lining

Molten metal
Molten-metal

hole
— Molten-metal
passage

3.4 Foundry Furnaces

(because slag floats on the surface of molten iron). The bottom of the cupola is
closed with drop doors to dump residual coke or metal and also to allow for mainte-
nance and repair of the furnace lining.

Operation. A bed of molding sand is first rammed on the bottom to a thickness of
about 6 inches (150 mm) or more. A bed of coke about 40 inches (1.0 m) thick is next
placed on the sand. The coke is then ignited, and air is blown at a lower-than-normal
rate. Next, the charge is fed into the cupola through the charging door. Many factors,
such as the charge composition, affect the final structure of the gray cast iron obtained.
Nevertheless, it can generally be stated that the charge is composed of 25 percent pig
iron, 50 percent gray cast-iron scrap, 10 percent steel scrap, 12 percent coke as fuel,
and 3 percent limestone as flux. These constituents form alternate layers of coke, lime-
stone, and metal. Sometimes, ferromanganese briquettes and inoculants are added to
the charge to control and improve the structure of the cast iron produced.

Direct Fuel-Fired Furnaces
(Reverberatory Furnaces)

The direct fuel-fired furnace, or reverberatory furnace, is used for the batch-type melt-
ing of bronze, brass, or malleable iron. The burners of the furnace are fired with pul-
verized coal or another liquid petroleum product. Figure 3.21 shows that the roof of
the reverberatory furnace reflects the flame onto the metal placed on the hearth, thus
heating the metal and melting it. The gaseous products of combustion leave the furnace
through the flue duct. The internal surface of the furnace is lined with fire bricks, and
there are charging and tap holes. When iron is melted, the fuel-air ratio is adjusted to
produce a completely white iron without free graphite flakes because they lower the
properties of the resulting malleable iron.

Crucible (Pot) Furnaces

Nonferrous metals like bronzes, brasses, aluminum, and zinc alloys are usually melted
in a crucible, or pot, furnace. Crucible furnaces are fired by liquid, gaseous, or pulver-
ized solid fuel. Figure 3.22 shows that the products of combustion in a crucible furnace

FIGURE 3.21

A reverberatory furnace

Burner

Charging
door

Casting and Foundry Work

FIGURE 3.22

A crucible furnace

Crucible

Flame

Refractory
brick

do not come in direct contact with the molten metal, thus enabling the production of
quality castings. Crucible furnaces can be stationary or tilting. When the stationary
type is employed, crucibles are lifted out by tongs and are then carried in shanks. On
the other hand, crucibles with long pouring lips are always used with the tilting type.
Crucibles are made of either refractory material or alloy steels (containing 25 per-
cent chromium). Refractory crucibles can be of the clay-graphite ceramic-bonded type
or the silicon-carbide carbon-bonded type. The first type is cheaper, while the second
one is more popular in industry. Ceramic crucibles are used when melting aluminum,
bronze, or gray cast iron, whereas brasses are melted in alloy steel crucibles. Different
alloys must not be melted in the same crucible to avoid contamination of the molten
metal.

Electric Furnaces

An electric furnace is usually used when there is a need to prevent the loss of any con-
stituent element from the alloy and when high purity and consistency of casting qual-
ity are required. An electric furnace is also employed when melting high-temperature
alloys. In all types of electric furnaces, whether they are electric-arc, resistance, or in-
duction furnaces, the electric energy is converted into heat.

Electric-arc furnace. The electric-arc furnace is the most commonly used type of
electric furnace. Figure 3.23 is a sketch of an electric-arc furnace. The heat generated
by an electric arc is transferred by direct radiation or by reflected radiation off the in-

FIGURE 3.23

An electric-arc furnace

Pouring
spout

Gear system (for rotating
rum at an adequate
e for pouring the
molten metal)

3.4 Foundry Furnaces

FIGURE 3.24

An electric-resistance
furnace

Refractory

lining

Molten
metal
Insulating

material

Pouring
spout

ternal lining of the furnace. The electric arc is generated about midway between two
graphite electrodes. In order to control the gap between the two electrodes and, ac-
cordingly, control the intensity of heat, one electrode is made stationary and the other
one movable. Electric-arc furnaces are used mainly for melting steels and, to a lesser
extent, gray cast iron and some nonferrous metals.

Resistance furnace. The resistance furnace is employed mainly for melting alu-
minum and its alloys. Figure 3.24 indicates the basic features of a typical resistance
furnace. The solid metal is placed on each of the two inclined hearths and is subjected
to heat radiation from the electric-resistance coils located above it. When the metal
melts, it flows down into a reservoir. The molten metal can be poured out through the
spout by tilting the whole furnace.

Induction furnace. The induction furnace has many advantages, including evenly dis-
tributed temperatures within the molten metal, flexibility, and the possibility of con-
trolling the atmosphere of the furnace. In addition, the motor effect of the
electromagnetic forces helps to stir the molten metal, thus producing more homoge-
neous composition. Induction furnaces are used to melt steel and aluminum alloys.
Figure 3.25 shows the construction of a typical induction furnace. It basically involves
an electric-induction coil that is built into the walls of the furnace. An alternating cur-
rent in the coil induces current in any metallic object that obstructs the electromagnetic

FIGURE 3.25

An electric-induction
furnace

Cover

Pouring
spout

Molten metal

(under stirring

action)

Electric-induction
coil (copper tubing)

68 3 Casting and Foundry Work

flux. Furnaces of both high- and low-frequency current are successfully used in indus-
try to induce alternating current in solid metal to melt it.

ASTING DEFECTS AND
DESIGN CONSIDERATIONS

Common Defects in Castings

In order to obtain a sound casting, it is necessary to control adequately the various fac-
tors affecting the casting process. Casting and pattern designs, molding procedure, and
melting and pouring of molten metal are among the factors affecting the soundness of
a casting. Following is a survey of the commonly experienced defects in castings.

Hot tears. Hot tears can appear on the surface or through cracks that initiate during
cooling of the casting. They usually are in locations where the metal is restrained from
shrinking freely, such as a thin wall connecting two heavy sections.

Cold shut. A cold shut is actually a surface of separation within the casting. It is be-
lieved to be caused by two "relatively cold" streams of molten metal meeting each
other at that surface.

Sand wash. A sand wash can be described as rough, irregular surfaces (hills and val-
leys) of the casting that result from erosion of the sand mold. This erosion is, in turn,
caused by the metal flow.

Sand blow. A sand blow is actually a surface cavity that takes the form of a very
smooth depression. It can be caused by insufficient venting, lack of permeability, or a
high percentage of humidity in the molding sand.

Scab. A scab is a rough "swollen" location in the casting that has some sand embed-
ded in it. Such a defect is usually encountered when the molding sand is too fine or too
heavily rammed.

Shrinkage porosity (or cavity). A shrinkage porosity is a microscopic or macroscopic
hole formed by the shrinkage of spots of molten metal that are encapsulated by solid-
ified metal. It is usually caused by poor design of the casting.

Hard spots. Hard spots are hard, difficult-to-machine areas that can occur at different
locations.

Deviation of the chemical composition from the desired one. Deviation may be due
to the loss of a constituent element (or elements) during the melting operation. It may
also be caused by contamination of the molten metal.

Design Considerations

A product designer who selects casting as the primary manufacturing process should
make a design not only to serve the function (by being capable of withstanding the
loads and the environmental conditions to which it is going to be subjected during its

3.5 Casting Defects and Design Considerations

service life) but also to facilitate or favor the casting process. Following are some de-
sign considerations and guidelines.

Promote directional solidification. When designing the mold, be sure that the risers
are properly dimensioned and located to promote directional solidification of the cast-
ing toward the risers. In other words, the presence of large sections or heat masses in
locations distant from the risers should be avoided, and good rising practice as previ-
ously discussed should be followed. Use can also be made of chills to promote direc-
tional solidification. Failure to do so may result in shrinkage cavities (porosity) or
cracks in those large sections distant from the risers. It is also very important to re-
member that a riser will not feed a heavy section through a lighter section.

Ensure easy pattern drawing. Make sure that the pattern can easily be withdrawn from
the nonpermanent mold (this does not apply to investment casting). This can be
achieved through rational selection of the parting line as well as by providing appropri-
ate pattern draft wherever needed. In addition, undercuts or protruding bosses (espe-
cially if their axes do not fall within the parting plane) and the like should be avoided.
Nevertheless, remember that undercuts can be obtained, if necessary, by using cores.

Avoid the shortcomings of columnar solidification. Dendrites often start to form on
the cold surface of a mold and then grow to form a columnar casting structure. This
almost always results in planes of weakness at sharp corners, as illustrated in Fig-
ure 3.26a. Therefore, rounding the edges is a must for eliminating the development of
planes of weakness, as shown in Figure 3.26b. Rounded edges are also essential for
smooth laminar flow of the molten metal.

Avoid hot spots. Certain shapes, because of their effect on the rate of heat dissipation
during solidification, tend to promote the formation of shrinkage cavities. This is al-
ways the case at any particular location where the rate of solidification is slower than
that at the surrounding regions of the casting. The rate of solidification (and the rate
of heat dissipation to start with) is slower at locations having a low ratio of surface
area to volume. Such locations are usually referred to as hot spots in foundry work.
Unless precautions are taken during the design phase, hot spots and, consequently,
shrinkage cavities are likely to occur at the L, T, V, Y, and + junctions, as illustrated in
Figure 3.27a. Shrinkage cavities can be avoided by modifying the design, as shown in

FIGURE 3.26

Columnar solidification
and planes of
weakness: (a) poor
design (sharp corner);
(b) rounded edges to
eliminate planes of
weakness

(a)

(b)

3 Casting and Foundry Work

FIGURE 3.27

Hot spots: (a) poor
design, yielding hot
spots; (b) better
design, eliminating hot
spots

rcn

<2X Cored
holes

(b)

Figure 3.27b. Also, it is always advisable to avoid abrupt changes in sections and to
use taper (i.e., make the change gradual), together with generous radii, to join thin to
heavy sections, as shown in Figure 3.28.

Avoid the causes of hot tears. Hot tears are casting defects caused by tensile stresses
as a result of restraining a part of the casting. Figure 3.29a and b shows locations where
hot tears can occur and a recommended design that would eliminate their formation.

Distribute the masses of a section to save material. Cast metals are generally
weaker in tension in comparison with their compressive strengths. Nonetheless, the
casting process offers the designer the flexibility of distributing the masses of a section
with a freedom not readily available when other manufacturing processes are em-
ployed. Therefore, when preparing a design of a casting, try to distribute masses in
such a manner as to lower the magnitude of tensile stresses in highly loaded areas of
the cross section and to reduce material in lightly loaded areas. As can be seen in Fig-
ure 3.30, a T section or an I beam is more advantageous than just a round or square
one when designing a beam that is to be subjected to bending.

Avoid thicknesses lower than the recommended minimum section thickness. The

minimum thickness to which a section of a casting can be designed depends upon such
factors as the material, the size, and the shape of the casting as well as the specific

FIGURE 3.28

Avoiding abrupt
changes in sections

@=<o)

Poor

Better

3.5 Casting Defects and Design Considerations

FIGURE 3.29

Hot tears: (a) a casting
design that promotes
hot tears; (b)
recommended design
to eliminate hot tears

(a)

(b)

casting process employed (i.e., sand casting, die casting, etc.). In other words, strength
and rigidity calculations may prove a thin section to be sufficient, but casting consid-
erations may require adopting a higher value for the thickness so that the cast sections
will fill out completely. This is a consequence of the fact that a molten metal cools
very rapidly as it enters the mold and may become too cold to fill a thin section far
from the gate. A minimum thickness of 0.25 inch (6 mm) is suggested for design use
when conventional steel casting techniques are employed, but wall thicknesses of
0.060 inch (1.5 mm) are quite common for investment castings. Figure 3.31 indicates
the relationship between the minimum thickness of a section and its largest dimension.
It should be pointed out that for a given thickness, steel flows best in a narrow rather
than in a wide web. For cast-iron and nonferrous castings, the recommended values for
minimum thicknesses are much lower than those for steel castings having the same
shape and dimensions.

Strive to make small projections in a large casting separate. As can be seen in Fig-
ure 3.32a, a small projection may be subjected to more accidental knocks than a large

FIGURE 3.30

Distribution of masses
to reduce weight

nbending

Beam

cr7

T section

Stress distribution

I beam

Stress distribution

Very low
stress area
(material not
fully utilized)

Square bar

Stress distribution

3 Casting and Foundry Work

FIGURE 3.31

0.375

Minimum thickness of

0 25

cast steel sections as

a function of their

</>

0.125

largest dimension
(Adapted from Steel
Castings Handbook,

0
0.750

5th ed. Rocky River,
Ohio: Steel Founders

E
c
2

0.500

Society of America,
1980)

0.250
0

_ ^^^^-^

Length of section (cm)

100

200

300
i

400 500 600 700
i i i i

I 1

i i

E
E,

30 »

20 £

E
10 I

50 100 150 200

Length of section (in.)

250

300

casting, and if it gets broken, the whole casting will be scrapped. It is, therefore, highly
recommended to make the small projection separate and attach it to the large casting
by an appropriate mechanical joining method, as shown in Figure 3.32b.

Strive to restrict machined surfaces. Whereas some castings are used in their en-
tirely as-cast condition, some others may require one or more machining operations. It
is the task of the designer to ensure that machining is performed only on areas where
it is absolutely necessary. An example of cases where the machining needed involves
bearing surfaces is shown in Figure 3.33.

Use reinforcement ribs to improve the rigidity of thin, large webs. A common use of
brackets or reinforcement ribs is to provide rigidity to thin, large webs (or the like) as
an alternative to increasing the thickness of the webs. The ribs should be as thin as
possible (i.e., minimum permissible thickness) and should also be staggered, as shown
in Figure 3.34. Always remember that parabolic ribs are better than straight ribs in
terms of economy and uniformity of stress.

Consider the use of cast-weld construction to eliminate costly cored design. The de-
sign of some products necessitates the use of complicated steel- wire-reinforced cores
that are difficult to reach and remove after casting, thus leaving the surfaces unclean.
An example, a steam ring, is shown in Figure 3.35a. The alternative design would be
to employ a simple cut plate that is welded into the casting to produce the cast-weld
construction shown in Figure 3.35b.

3.6 CLEANING, TESTING, AND
INSPECTION OF CASTINGS

Cleaning

The cleaning process involves the removal of the molding sand adhering to a casting.
It also includes the elimination of gates, runners, and risers. Generally, surface clean-
ing can be carried out in rotary separators or by employing sand-blasting and/or metal-

3.6 Cleaning, Testing, and Inspection of Castings

FIGURE 3.32

Large casting with a
small projection: (a) as
an integral part; (b) two
separate parts

Small projection

lie shot-blasting machines. The latter two machines use sand particles or shots travel-
ing at high velocities onto the surface of the casting to loosen and remove the adher-
ing sand. As you may expect, these machines are particularly suitable when cleaning
medium and heavy castings. On the other hand, rotary separators are advantageous for
cleaning light castings. A separator is actually a long, large-diameter drum that ro-
tates around its horizontal axis into which the castings are loaded together with jack

3 Casting and Foundry Work

FIGURE 3.33

Restriction of surfaces
to be machined

FIGURE 3.34

Use of reinforcement
ribs

1 in.
(25 mm)

FIGURE 3.35

The design of a steam
ring: (a) cast
construction; (b) cast-
weld construction

7777/AW////////A'.

/////////////////////// / TT7
(a)

7777/7^) /////// 77^777. _

>1\\\\\\\\\\\\\\\\\\\\\\\\\

Plate welded
(b)

3.7 Castability (Fluidity) 75

stars made of white cast iron. A further advantage of rotary separators is that they au-
tomatically break off gate-and-runner systems and, often, risers.

Testing and Inspection

Like any other manufactured parts, castings must be subjected to thorough quality con-
trol in order to separate defective products and to reduce the percentage of rejects
through identifying the defects and tracing their sources. Following are some of the
commonly used tests and inspection methods.

Testing of the mechanical properties of the casting. Standard tension and hardness
tests are carried out to determine the mechanical properties of the metal of the casting
in order to make sure that they conform to the specifications.

Inspection of the dimensions. Dimensions must fall within the specified limits.
Therefore, measuring tools and different kinds of gages (e.g., snap, progressive, plug,
template) are used to check that the dimensions conform to the blueprint.

Visual examination. Visual inspection is used to reveal only very clear defects. How-
ever, it is still commonly used in foundries.

Hydraulic leak testing. The hydraulic leak test is used to detect microscopic shrink-
age porosity. Various penetrants and testing methods are now available. Details are
given in the American Society for Testing and Materials (ASTM) standards, designa-
tion E165.

Nondestructive testing. There are several nondestructive testing methods that detect
microscopic and hair cracks. They involve ultrasonic testing, magnetic particle inspec-
tion, eddy current testing, and radiography.

Testing for metal composition. Several methods are employed to determine the chem-
ical composition accurately and to assure product quality. The classical method used to
be "wet analysis" (i.e., employing acids and reagents in accurate chemical analysis).
However, because this method is time-consuming, it is being replaced by methods like
emission spectroscropy, X-ray fluorescence, and atomic absorption spectroscopy.

CASTABILITY (FLUIDITY)

The ability of the molten metal to flow easily without premature solidification is a
major factor in determining the proper filling of the mold cavity. This important prop-
erty is referred to as castability or, more commonly, fluidity. The higher the fluidity of
a molten metal, the easier it is for that molten metal to fill thin grooves in the mold and
exactly reproduce the shape of the mold cavity, thereby successfully producing cast-
ings with thinner sections. Poor fluidity leads to casting defects such as incomplete fill-
ing or misruns, especially in the thinner sections of a casting. Because fluidity is
dependent mainly upon the viscosity of the molten metal, it is clear that higher tem-
peratures improve the fluidity of molten metal and alloys, whereas the presence of im-
purities and nonmetallic inclusions adversely affects it.

3 Casting and Foundry Work

FIGURE 3.36

Details of the test for
measuring fluidity

0.3 inch

«-i*A7I[a3

inch

Magnified
section A-A

Several attempts have been made to quantify and measure the fluidity of metals.
A commonly used standard test involves pouring the molten metal into a basin so that
it flows along a spiral channel of a particular cross section, as shown in Figure 3.36.
Both the basin and the channel are molded in sand, and the fluidity value is indicated
by the distance traveled by the molten metal before it solidifies in the spiral channel.

Review Questions

1. What is meant by the word casting ?

2. What are the constituents of green molding
sand?

3. List some of the important properties that green
sand must possess.

4. What is a flask? What is its function? List the
parts that form a flask.

5. Explain the meaning of the word pattern.

6. List some of the materials used in making pat-
terns.

7. List the different types of permanent patterns
used in foundries.

8. What are the different pattern allowances? Dis-
cuss the function of each.

9. What are cores? How are they made?

10. What is meant by a gating system ? What func-
tions does it serve?

11. What are the components of a gating system?

12. What are risers? What function do they serve?

13. List the various green sand properties and dis-
cuss each briefly.

14. Why should weights be located on the cope in
pit molding?

Chapter 3 Review Questions

15. List the various molding machines and discuss
the operation of each briefly.

16. Explain sand conditioning and how it is done.

17. What advantages does dry sand molding have
over green sand molding?

18. When are cement-bonded sand molds recom-
mended?

19. What is the main advantage of the carbon diox-
ide process for molding?

20. What metals can be cast in plaster molds?

21. When are loam molds used?

22. Describe shell molding. What are its advan-
tages?

23. When are ceramic molds recommended?

24. Explain investment casting and why it is some-
times called the lost-wax process.

25. Name a metal that should be cast in a graphite
mold.

26. What are the advantages of employing perma-
nent molds? Why?

27. Can molten metals be cast directly into cavities
of cold permanent molds? Why?

28. What is the main difference between the hot-
chamber and the cold-chamber methods of die
casting?

29. List some metals that you think can be cast by
the hot-chamber method. Justify your answer.

30. List some metals that you think can be cast by
the cold-chamber method. Justify your answer.

31. What are the types of centrifugal casting?

32. Differentiate between the different types of
centrifugal casting and discuss the advantages
and shortcomings of each type.

33. What are the products that can be manufactured
by continuous casting?

34. What does the continuous casting process in-
volve?

35. Discuss some advantages of the continuous
casting process.

36. What does the V-process involve?

37. List some of the merits and advantages of the
V-process.

38. Discuss some of the problems encountered in
casting steels.

39. What precautions should be taken to eliminate
the problems in casting steels?

40. What is gray cast iron?

41. Discuss some of the properties that make gray
cast iron attractive for some engineering appli-
cations.

42. Why are inoculants added to gray cast iron?

43. Differentiate between gray cast iron and white
cast iron.

44. What is meant by compacted-graphite cast iron.

45. What is ductile cast iron? How can it be ob-
tained?

46. What is malleable cast iron? How can it be
obtained? What are the limitations on produc-
ing it?

47. List some alloying elements that are added to
cast iron. List some applications for alloyed
cast iron.

48. What are the problems caused by hydrogen
when melting and casting aluminum and how
can these problems be eliminated?

49. What are the sources of hydrogen when melting
aluminum?

50. List some cast aluminum alloys and discuss
their applications.

51. How are cast copper alloys classified?

52. What is meant by a deoxidizer? Give an ex-
ample.

53. List some of the characteristics and applica-
tions of cast zinc alloys.

54. List some of the characteristics and applica-
tions of cast magnesium alloys.

55. For what purpose is the cupola furnace used?

56. Describe briefly the operation and charge of the
cupola furnace.

3 Casting and Foundry Work

57.

For what purpose is the reverberatory furnace
used?

List some of the metals that can be melted in
crucible furnaces.

59. What are the main differences in construction
between the stationary and the tilting crucible
furnaces?

60. List the different types of electric furnaces and
mention the principles of operation in each case.

61. List the main advantages and applications of
electric furnaces.

62. List some of the common defects of castings
and discuss the possible causes of each defect.

63. List and discuss the main design considerations
for castings.

64. List and discuss the various testing and inspec-
tion methods used for the quality control of
castings.

Design Example

PROBLEM

Your company has received an order to manufacture wrenches for loosening and tight-
ening nuts and bolts of large machines. The plant of the company involves a foundry
and a machining workshop with a few basic machine tools. Here are the details of the
order:

Lot size:
Nut size:
Required torque:

500 wrenches

2 inches (50 mm)

about 20 lb ft (27.12 N-m)

You are required to provide a design and a production plan (see the explanation of the
word design in the design projects section that appears later).

Solution

Before we start solving this design problem, we should make some assumptions. For
instance, consider the force that can be generated by the ordinary human hand. It will
allow us to determine the length of the wrench using the following equation:

T=Fxt

where: T is the torque
F is the force
(, is the length

As can be seen from the equation, a low value of F would make the length large and
thus make the handling of the wrench impractical because of the weight. On the other
hand, a high value of F is not practical and may not be generated by an ordinary per-
son. Let us take F = 15 pounds. Therefore,

Chapter 3 Design Example 79

^ = T7= 1-33 feet

Apparently, the force acts at the middle of the fist, and we have to add a couple of
inches for proper holding:

length of wrench = 1 8 inches

Let us now design the section where the maximum bending moment occurs. You
can assume some dimensions and determine the stress, which will serve as a guide in
selecting material. Take the section as shown in Figure 3.37a. The moment of inertia
of the section is

/ = -j^(0.25)(0.75)3 + 2

— (0.375)(0.25)3 + 0.375 x 0.25 x 0.5

= 0.008789 + 0.00098 + 0.046875
= 0.056655 in.4

Note that the minimum thickness for steel casting was adhered to. Now, determine the
stress:

20 x 12 x 1.25 .,.. „ „ 2

max. stress = = 2648 lb/in.

2 x 0.056655

That value is very low, and we should try to reduce the section and save material. It is
always a good idea to make use of spreadsheets to change the dimensions and get the
stresses acting in each case. Now, take the section as shown in Figure 3.37b:

/ = ^(0.25)(0.5)3 + 2

— (0.375)(0.25)3 + 0.375 x 0.25 x 0.3752

= 0.002604 + 0.00098 + 0.026367
= 0.038771188 in.4

20 x 12 1.25 ,0,01ur 2

max. stress = — — — x — —— = 3868 lb/in.

0.038771188 2

As can be seen, we took the minimum thickness to be 0.25 inch, which is the recom-
mended value for conventional castings of steels.

The material should be low-carbon steel having 0.25 percent carbon in order to
possess enough ductility. Also, the steel should be thoroughly killed. A recommended
material is ASTM A27-77, grade U60-30, which has a yield strength of 30 ksi. When
taking a factor of safety of 4, the allowable stress would be 7500 lb/in. , which is
higher than the obtained value of the working stress.

Now, in order to calculate the thickness of the wrench, let us calculate the bearing
stress on the nut. A reasonable estimate of the force on the surface of the nut is

20 x 12

= 320 pounds

0.75 F

3 Casting and Foundry Work

FIGURE 3.37

Cross section of the
wrench: (a) first
attempt; (b) second
attempt

0.25 inch

0.75 inch

0.25 inch

0.5 inch

0.25 inch

This is based on the assumption that the torque is replaced by two opposite forces hav-
ing a displacement of 0.75 inch between the lines of action. Thus,

bearing stress =

320

= 7500

0.75 x t
t = 0.056 inch

Take it as 0.5 inch to facilitate casting the part.

Because all dimensions are known, a detailed design can be prepared, as shown in
Figure 3.38. Notice the surface finish marks indicating the surfaces to be machined.

FIGURE 3.38

A wrench manufactured by casting

f?= 1.0 inch

1.4 inch

'ncn

0.5 inch

Djo.

375 inch

Chapter 3 Design Projects 81

As previously mentioned, conventional sand casting is to be employed, using a
cope-and-drag pattern plate to cast two wrenches per flask. We can use a single down
sprue to feed the narrow end of the wrench and a riser at the other end. The parting
line will pass through the web of the / section.

Design Projects

Whenever the word design is mentioned hereafter, you should provide, at least, the
following:

• Two neatly dimensioned graphical projections of the product (i.e., a blueprint ready
to be released to the workshop for actual production), including fits (if applicable),
tolerances, surface finish marks, and so on

• Material selection with rational justification

• Selection of the specific manufacturing processes required, as well as their se-
quence in detail

• Simple but necessary calculations to check the stresses at the critical sections

1. Design a bracket for a screw C-clamp that has the following characteristics:

Maximum clamping force: 22 pounds (100 N)

Clamping gap: 3 inches (7.5 cm)
Distance between centerline of screw

and inner surface of bracket: 2 inches (5 cm)

Root diameter of screw: 0.25 inch (6 mm)

Assume that manufacturing is by casting and that production volume is 4000
pieces.

2. Design a flat pulley. Its outer diameter is 36 inches (90 cm), and it is to be
mounted on a shaft that is 2x/i inches (6.25 cm) in diameter. Its width is 10 inches
(25 cm), and it has to transmit a torque of 3000 lb ft (4000 Nm). Assume that 500
pieces are required. Will the design change if only 3 pieces are required?

3. A connecting lever has two short bosses, each at one of its ends and each with a
vertical hole that is 3/4 inch (19 mm) in diameter. The lever is straight, and the
horizontal distance between the centers of the holes is 8 inches (200 mm). The
lever during functioning is subjected to a bending moment of 50 lb ft (67.8 Nm)
that acts in the plane formed by the two vertical axes. Provide a detailed design
for this lever if it is to be produced by casting and

a. When only 100 pieces are required

b. When 1 0,000 pieces are required

82 3 Casting and Foundry Work

4. Design a micrometer frame for each of the following cases:

a. The gap of the micrometer is 1.0 inch (25 mm), and the distance from the axis
of the barrel to the inner side of the frame is 1 .5 inches (37.5 mm). The maxi-
mum load on the anvil is 22 lb (100 N).

b. The gap of the micrometer is 6 inches (150 mm), and the distance from the axis
of the barrel to the inner side of the frame is 4.0 inches (100 mm). The maxi-
mum load on the anvil is 22 lb (100 N).

Assume that production volume is 4000 pieces and that one of the various casting
processes is used.

TIP: Base your design on rigidity. The maximum deflection must not exceed
0.1 of the smallest reading of the micrometer.

5. A pulley transmits a torque of 600 lb ft (813.6 Nm) to a shaft that is \XA inches
(31 mm) in diameter. The outer diameter of the pulley is 10 inches (250 mm), and
it is to be driven by a flat belt that is 2 inches (50 mm) in width. Design this pul-
ley if it is to be manufactured by casting and 500 pieces are required.

6. Design a hydraulic jack capable of lifting 1 ton and having a stroke of 6 inches
(150 mm). The jack is operated by a manual displacement (plunger) pump that
pumps oil from a reservoir into the high-pressure cylinder through two spring-
actuated nonreturn valves to push the ram upward. The reservoir and the high-
pressure cylinder are also connected by a conduit, but the flow of oil is obstructed
by a screw that, when unscrewed, relieves the pressure of the cylinder by allow-
ing high-pressure oil to flow back into the reservoir and the ram then to be pushed
downward. Provide a workshop drawing for each component, as well as an as-
sembly drawing for the jack. Steel balls and springs are to be purchased. Assume
production volume is 5000 pieces.

7. Design a table for the machine shop. That table should be 4 feet (1.2 m) in height,
with a surface area of 3 by 3 feet (0.90 by 0.9 m), and should be able to carry a
load of half a ton. Assume production volume is 2000 pieces.

8. Design a little wrench for loosening and tightening nuts and bolts of a bicycle. The
nut size is 5/8 inch (15 mm), and the required torque is about 1.0 lb ft (1.356
Nm). Assume production volume is 10,000 pieces.

9. A straight-toothed spur-gear wheel transmits 1200 lb ft (1627 Nm) of torque to a
steel shaft that is 2 inches (50 mm) in diameter. The pitch diameter of the gear is
8 inches (200 mm), its width is 3 inches (75 mm), and the base diameter is 7.5
inches (187.5 mm). Design this gear's blank. Assume production volume is 4000
pieces.

10. Design a frame for an open-arch (C-type) screw press that can deliver a load of up
to 2 tons. The open gap is 2 feet (600 mm), and the bed on which workpieces are
placed is 12 by 12 inches (300 by 300 mm). Assume that the base diameter of the
screw thread is 1 Vi inches (37.5 mm).

Chapter 3 Design Projects 83

11. Design a hydraulic cylinder for earth-moving equipment. It can generate a maxi-
mum force of 2 tons and has a stroke of 4 feet (1200 mm). Although the maximum
force is generated only when the plunger rod is moving out, the cylinder is dou-
ble acting and generates a force of 1 ton during its return stroke. Expected pro-
duction volume is 2000 pieces, and the pistons, oil rings, and so on, are going to
be purchased from vendors.

12. Design a safety valve to be mounted on a high-pressure steam boiler. The pipe on
which it will be mounted has a bore diameter of 2 inches (50 mm). The pressure
inside the boiler should not exceed 50 folds of the atmospheric pressure. Expected
production volume is 5000 pieces, and the stems, springs, bolts, and gaskets are
going to be purchased from vendors.

Chapter 4

Inlng of Metals

^s— "*^&

IVETING

INTRODUCTION

When two parts of metal are to be attached together, the resulting joint can be
made dismountable (using screws and the like), or it can be made permanent
by employing riveting, welding, or brazing processes. The design of dismount-
able joints falls beyond the scope of this text and is covered in machine de-
sign. It is, therefore, the aim of this chapter to discuss the design and
production of permanent joints when various technologies and methods are ap-
plied. Because the same equipment used in welding is also sometimes em-
ployed in the cutting of plates, thermal cutting processes will also be
discussed in this chapter.

The process of riveting involves inserting a ductile metal pin through holes in two or
more sheet metals and then forming over (heading) the ends of the metal pin so as to
secure the sheet metals firmly together. This process can be performed either cold or
hot, and each rivet is usually provided with one preformed head. Figure 4.1a and b in-
dicates the sequence of operations in riveting, while Figure 4.2 illustrates different
shapes of preformed rivet heads.

ELDING

Welding is the joining of two or more pieces of metal by creating atom-to-atom bonds
between the adjacent surfaces through the application of heat, pressure, or both. In
order for a welding technique to be industrially applicable, it must be reasonable in
cost, yield reproducible or consistent weld quality, and, more importantly, produce

4.2 Welding

FIGURE 4.1

Sequence of operations
in riveting: (a) flat-head
rivet; (b) regular rivet

Hammering

vzz& m%,

w&. ^m

Lower

tool

— V—

(a)

(b)

joints with properties comparable to those of the base material. Various welding tech-
niques have been developed that are aimed at achieving these three goals. However, no
matter what welding method is used, the interface between the original two parts must
disappear if a strong joint is to be obtained. Before we discuss the different methods
employed to make those surfaces disappear, let us discuss joint design and preparation.

Joint Design and Preparation

A weld joint must be designed to withstand the forces to which it is going to be sub-
jected during its service life. Therefore, the joint design is determined by the type and
magnitude of the loading that is expected to act on the weldment. In other words, se-
lection of the type of joint has to be made primarily on the basis of load requirement.
As Figure 4.3a through e shows, there are five types of weld joints: butt, lap, corner,
T, and edge. Following is a discussion of each of these different types of joints.

Butt joint. The butt joint involves welding the edges or end faces of the two original
parts, as shown in Figure 4.3a. Therefore, the two parts must be aligned in the same
plane. Usually, when the thickness of the parts falls between 1/8 and 3/8 inch (about 3
and 9 mm), the two parts are welded without any edge preparation. This type of weld
is referred to as a square weld and can be either single or double, depending upon the
thickness of the metal, as shown in Figure 4.4a. As can be seen in Figure 4.4b through
e, the edges of thicker parts should be prepared with single or double bevels or V-, J-,
or U-grooves to allow adequate access to the root of the joint. Usually, it is recom-
mended to adopt the single or double U-groove when the thickness of the parts is more
than 0.8 inch (20 mm).

Lap joint. We can see in Figure 4.3b that the lap joint is produced by fillet welding
overlapping members; the amount of overlap is normally taken to be about three to

FIGURE 4.2

Different shapes of
preformed rivet heads

^Z7 c

4 Joining of Metals

FIGURE 4.3

Types of weld joints:
(a) butt joint; (b) lap
joint; (c) corner joint;
(d) T-joint; (e) edge joint

1 \

(b)

CZlAZZ]

(d)

(e)

five times the thickness of the member. The fillet weld can be continuous and may also
be of the plug or slot type, as shown in Figure 4.5.

Corner joint. Figure 4.3c illustrates the corner joint, which can be welded with or
without edge preparation (see Figure 4.4 for the various possible edge preparations).

T-joint. The T-joint is shown in Figure 4.3d. T-joints that will be subjected to light
static loads may not require edge preparation. On the other hand, edge preparations
(again see Figure 4.4) are often necessary for greater metal thicknesses or when the
joint is to be subjected to relatively high, alternating, or impulsive loading.

Edge joint. The edge joint is usually used when welding thin sheets of metal with a
thickness of up to 1/8 inch (3 mm). Notice in Figure 4.3e that the edges of the mem-
bers must be bent before the welding process is carried out.

FIGURE 4.4

Different edge
preparation for butt
welding: (a) square; (b)
bevel; (c) V-groove; (d)
J-groove; (e) U-groove

(Single)
square

(Double)
square

Single Single Single Single

(a)

Double
(b)

Double
(c)

Double
(d)

Double
(e)

FIGURE 4.5

Basic types of fusion
lap welds

o ©

Fillet

Plug

Slot

4.2 Welding

FIGURE 4.6

Weld symbols

j^y

Groove weld
symbol

rry

Fillet
Groove welds:

Square

Plug or slot

Back or backing Spot or projection Seam

Vnv

Bevel

Weld Symbols and Identification

Figure 4.6 shows the different weld symbols, whereas Figure 4.7 shows the standard
identification of welds employed in design drawings.

Classification of the Welding Processes

Different methods can be used for classifying industrial welding processes. Each
method is employed to form groups of welding processes, with each group having
something in common. For instance, welding processes can be classified according to
the source of energy required to accomplish welding. In such a case, it is obvious that
there are four main groups: mechanical, electrical, chemical, or optical. Welding
processes can also be classified by the degree of automation adopted, which yields
three groups: manual, semiautomatic, and automatic. The most commonly used
method of classification is according to the state of the metal at the locations being

FIGURE 4.7

Standard identification
of welds

Basic weld symbol
or detail reference

Size; size or strength
for resistance welds

Reference line

Root opening; depth

of filling for plug

and slot welds

Specification, process,
or other reference

Tail (may be omitted

when reference

is not used)

Basic weld symbol
or detail reference

Finish
symbol

Contour
symbol

Groove angle included
angle or countersink
for plug weld
Pitch (center-to-center
spacing) of welds
Arrow connecting reference
line to arrow side of joint,
to grooved member, or both

(N)

Number of spots
or projection welds

Elements in this area

remain as shown

when tail and arrow

are reversed

Field weld
symbol

Weld

all around

symbol

88 4 Joining of Metals

welded, thus splitting the welding processes into two main categories: pressure weld-
ing and fusion welding. We now discuss each of these two categories in detail.

Pressure Welding Processes

Pressure welding involves processes in which the application of external pressure is
indispensable to the production of weld joints formed either at temperatures below the
melting point (solid-state welding) or at temperatures above the melting point (fusion
welding). In both cases, it is important to have very close contact between the atoms
of the parts that are to be joined. The atoms must be moved together to a distance that
is equal to or less than the equilibrium interatomic-separation distance. Unfortunately,
there are two obstacles that must be overcome so that successful pressure welding can
be carried out and a sound weldment can be obtained. First, surfaces are not flat when
viewed on a microscopic scale. Consequently, intimate contact can be achieved only
where peaks meet peaks, as can be seen in Figure 4.8, and the number of bonds would
not be enough to produce a strong welded joint. Second, the surfaces of metals are usu-
ally covered with oxide films that inhibit direct contact between the two metal parts to
be welded. Therefore, those oxide and nonmetallic films must be removed (cleaned
with a wire brush) before welding in order to ensure a strong welded joint. Pressure
welding processes are applied primarily to metals possessing high ductility or those
whose ductility increases with increasing temperatures; thus, the peaks that keep the
surfaces of the two metallic members apart are leveled out under the action of me-
chanical stresses or the combined effect of high temperatures and mechanical stresses.
In fact, a wide variety of pressure welding processes are used in industry. The com-
monly used ones are listed in Figure 4.9.

Cold-pressure welding. Cold-pressure welding is a kind of solid-state welding used
for joining sheets, wires, and small electric components. As previously discussed, the
surfaces to be welded must be cleaned with a wire brush to remove the oxide film
and must be carefully degreased before welding. As Figure 4.10 shows, a special
tool is used to produce localized plastic deformation, which results in coalescence be-
tween the two parts. This process, which can replace riveting, is usually followed by

FIGURE 4.8

A microscopic view of

two mating surfaces Surface 1

Surface 2

4.2 Welding

FIGURE 4.9

Classification of the
commonly used
pressure welding
processes

Pressure welding processes

Cold-pressure welding

* Cold-pressure welding
of sheets and wires

* Ultrasonic welding

* Explosive welding

Hot-pressure welding

Molten-metal bonding

* Percussion

* Resistance flash

* Resistance spot

* Resistance seam

* Resistance projection

* Thermit

Hot solid-state
pressure welding

* Diffusion bonding

* Friction welding

* Inertia welding

* Induction welding
« Resistance upset

(butt) welding

annealing of the welded joint. Figure 4.10 also shows that recrystallization takes place
during the annealing operation. This is added to diffusion, which finally results in com-
plete disappearance of the interface between the two parts.

Cold-pressure welding of wires is performed by means of a special-purpose ma-
chine. Figure 4. 1 1 illustrates the steps involved in this process. As can be seen, the
wires' ends are clamped and pressed repeatedly against each other in order to ensure
adequate plastic deformation. The excess upset metal is then trimmed by the sharp
edges of the gripping jaws. This technique is used when welding wires of nonferrous
metals such as aluminum, copper, or aluminum-copper alloys.

Explosive welding. Explosive welding is another technique that produces solid-state
joints and, therefore, eliminates the problems associated with fusion welding methods,
like the heat-affected zone and the microstructural changes. The process is based on
using high explosives to generate extremely high pressures that are, in turn, used to
combine flat plates or cylindrical shapes metallurgically. Joints of dissimilar metals
and/or those that are extremely difficult to combine using conventional methods can
easily be produced by explosive welding.

During explosive welding, a jet of soft (or fluidlike) metal is formed (on a micro-
scopic scale) and breaks the oxide film barrier to bring the two metal parts into inti-
mate contact. That metal jet is also responsible for the typical wavy interface between

FIGURE 4.10

Cold-pressure welding
of sheets

Welds

Punch

Recrystallization

r?i

Punch

Welded joint after
deformation

Welded joint after
annealing

4 Joining of Metals

FIGURE 4.11

Cold-pressure welding
of wires

Clamps

Upset metal

J2,

S £

After trimming

the two metal parts, thus creating mechanical interlocking between them and, finally,
resulting in a strong bond. Figure 4. 1 2 illustrates an arrangement for explosive welding
two flat plates, and Figure 4.13 is a magnified sketch of the wavy interface between
explosively welded parts.

FIGURE 4.12

An arrangement for
explosive welding two
flat plates

Detonator

Explosive
layer

^Ml^M^

Plate 1

Plate 2

plate

Explosive welding and explosive cladding are popular in the manufacture of heat
exchangers and chemical-processing equipment. Armored and reinforced composites
with a metal matrix are also produced by explosive welding. Nevertheless, a clear
limitation is that the process cannot be used successfully for welding hard, brittle met-
als. Research is being carried out in this area, and new applications are continuously
introduced.

Ultrasonic welding. The ultrasonic welding method of solid-state welding is com-
monly used for joining thin sheets or wires of similar or dissimilar metals in order
to obtain lap-type joints. Mechanical vibratory energy with ultrasonic frequencies is
applied along the interfacial plane of the joint, while a nominal static stress is applied,
normal to that interface, to clamp the two components together. Oscillating shear

FIGURE 4.13

A sketch of the wavy
interface between
explosively welded
parts

Interface

4.2 Welding 91

stresses are, therefore, initiated at the interface and disperse surface films, allowing in-
timate contact between the two metals, and, consequently, producing a strong joint. Ul-
trasonic welding does not involve the application of high pressures or temperatures
and is accomplished within a short time. Therefore, this process is especially suitable
for automation and has found widespread application in the electrical and microelec-
tronics industries in the welding of thin metal foils for packaging and splicing and in
the joining of dissimilar materials in the fabrication of nuclear reactor components. It
must be noted, however, that the process is restricted to joining thin sheets or fine
wires. Nevertheless, this restriction applies only to thinner pieces, and the process is
often used in welding thin foils to thicker sheets.

Different types of ultrasonic welding machines are available, each constructed to
produce a certain type of weld, such as spot, line, continuous seam, or ring. A sketch
of a spot-type welding machine that is commonly used in welding microcircuit ele-
ments is illustrated in Figure 4.14. As we can see, the machine consists basically of a
frequency convenor that transforms the standard 60-Hz (or 50-Hz in Europe) electric
current into a high-frequency current (with a fixed frequency in the range of 15 to 75
kHz), a transducer that converts the electrical power into elastic mechanical vibrations,
and a horn that magnifies the amplitude of these vibrations and delivers them to the
weld zone. Other associated elements include the anvil, a force-application and clamp-
ing device, a sonotrode (as compared with the electrode in resistance welding), and ap-
propriate controls to set up optimal values for the process variables, such as vibratory
power and weld time.

Friction welding. In friction welding, a type of hot solid-state welding, the parts to be
welded are tightly clamped, one in a stationary chuck and the other in a rotatable
chuck that is mounted on a spindle. External power is employed to drive the spindle at
a constant speed, with the two parts in contact under slight pressure. Kinetic energy is
converted to frictional heat at the interface. When the mating edges of the workpieces
attain a suitable temperature (in the forging range) that permits easy plastic flow, the
spindle rotation is halted, and high axial pressure is applied to plastically deform the
metal, obtain intimate contact, and produce a strong, solid weld. This is clearly shown
in Figure 4.15, which indicates the stages involved in friction welding.

Several advantages have been claimed for the friction welding process. These in-
clude simplicity, high efficiency of energy utilization, and the ability to join similar as

FIGURE 4.14 Clamping

A sketch of an Transducer H°m *?*

ultrasonic spot-type
welding machine

Workpieces

92 4 Joining of Metals

FIGURE 4.15

Stages involved in
friction welding

Low
thrust

IBEE3 -EEEEE3- E--EE3-

OAT

Rotating Heating stage Upsetting stage

well as dissimilar metal combinations that cannot be joined by conventional welding
methods (e.g., aluminum to steel or aluminum to copper). Also, since contaminants
and oxide films are carried away from the weld area where grain refinement takes
place, a sound bond is obtained and usually has the same strength as the base metal.
Nevertheless, a major limitation of the process is that at least one of the two parts to
be joined must be a body of revolution around the axis of rotation (like a round bar or
tube). A further limitation is that only forgeable metals that do not suffer from hot
shortness can successfully be friction welded. Also, care must be taken during welding
to ensure squareness of the edges of workpieces as well as concentricity of round bars
or tubes.

Inertia welding. Inertia welding is a version of friction welding that is recom-
mended for larger workpieces or where high-strength alloys (i.e., superalloys) are to
be joined together. Inertia welding, as the name suggests, efficiently utilizes the ki-
netic energy stored in a rotating flywheel as a source for heating and for much of the
forging of the weld. As is the case with friction welding, the two workpieces to
be inertia welded are clamped tightly in stationary and rotatable chucks, the differ-
ence being that the rotatable chuck is rigidly coupled to a flywheel in the case of
inertia welding. The process involves rotating the flywheel at a predetermined angu-
lar velocity and then converting the kinetic energy of the freely rotating flywheel to
frictional heat at the weld interface by applying an axial load to join the abutting
ends under controlled pressure. The process requires shorter welding time than that
taken in conventional friction welding, especially for larger workpieces. Examples of
inertia-welded components include hydraulic piston rods for agricultural machinery,
carbon steel shafts welded to superalloy turbocharger wheels, and bar stock welded
to small forgings.

Induction welding. As the name suggests, induction welding is based on the phe-
nomenon of induction. We know from physics (electricity and magnetism) that when
an electric current flows in an inductor coil, another electric current is induced in any
conductor that intersects with the magnetic flux. In induction welding, the source of
heat is the resistance, at the abutting workpieces' interface, to the flow of current in-
duced in the workpieces through an external induction coil. Figure 4.16 illustrates the
principles of induction welding. For efficient conversion of electrical energy into heat
energy, high-frequency current is employed, and the process is usually referred to as
high-frequency induction welding (HFIW). Frequencies in the range of 300 to 450 kHz
are commonly used in industry, although frequencies as low as 10 kHz are also in use.
It is always important to remember the "skin effect" when designing an induction-
welded joint. This effect refers to the fact that the electric current flows superficially
(i.e., near the surface). In fact, the depth of the layer through which the current flows
is dependent mainly upon the frequency and the electromagnetic properties of the

4.2 Welding

FIGURE 4.16

Principles of induction
welding

Workpiece-

^fiflflQflflV,

Coil ^I-
Flux

Workpiece

workpiece metal. Industrial applications of induction welding include butt welding of
pipes and continuous-seam welding for the manufacture of seamed pipes.

Thermit welding. Thermit welding makes use of an exothermic chemical reaction to
supply heat energy. That reaction involves the burning of thermit, which is a mixture
of fine aluminum powder and iron oxide in the form of rolling-mill scale, mixed at a
ratio of about 1 to 3 by weight. Although a temperature of 5400°F (3000°C) may be
attained as a result of the reaction, localized heating of the thermit mixture up to at
least 2400°F (1300°C) is essential in order to start the reaction, which can be given by
the following chemical formula:

8A1 + 3Fe304 -> 9Fe + 4A1203 + heat (4.1)

As we can see from the formula, the outcome is very pure molten iron and slag. In fact,
other oxides are also used to produce pure molten metals; these include chromium,
manganese, or vanadium, depending upon the parent metals to be welded.

Usually, the thermit welding process requires the application of pressure in order
to achieve proper coalescence between the parts to be joined. However, fusion thermit
welding is also used; it does not require the application of force. In this case, the re-
sulting molten metal is a metallurgical joining agent and not just a means for heating
the weld area.

Thermit welding is used in joining railroad rails, pipes, and thick steel sections, as
well as in repairing heavy castings. The procedure involves fitting a split-type refrac-
tory mold around the abutting surfaces to be welded, igniting the thermit mixture using
a primer (ignition powder) in a special crucible, and, finally, pouring the molten metal
(obtained as a result of the reaction) into the mold. Because the temperature of the
molten metal is about twice the melting point of steel, the heat input is enough to fuse
the abutting surfaces, which are usually pressed together to give a sound weld.

Diffusion bonding. Diffusion bonding is a solid-state welding method in which the
surfaces to be welded are cleaned and then maintained at elevated temperatures under
appropriate pressure for a long period of time. No fusion occurs, deformation is lim-
ited, and bonding takes place principally due to diffusion. As we know from metal-
lurgy, the process parameters are pressure, temperature, and time, and they should be
adjusted to achieve the desired results.

Butt welding. Butt welding belongs to the resistance welding group, which also con-
sists of the spot, seam, projection, percussion, and flash welding processes. All of these

4 Joining of Metals

FIGURE 4.17

Upset-butt welding

Clamping dies

Movable

Workpieces

Ac power supply

operate on the same principle, which involves heating the workpieces as a result of
being a part of a high-amperage electric circuit and then applying external pressure to
accomplish strong bonding. Consequently, all the resistance welding processes belong
to the larger, more general group of pressure welding; without the application of ex-
ternal pressure, the weld joint cannot be produced.

In butt welding, sometimes called upset-butt welding or just upset welding, the
parts are clamped and brought in solid contact, and low-voltage (1 to 3 V) alternating
current is switched on through the contact area, as illustrated in Figure 4.17. As a re-
sult of the heat generated, the metal in the weld zone assumes a plastic state (above the
solidus) and is gradually squeezed and expelled from the contact area. When enough
upset metal becomes evident, the current is switched off and the welded parts are re-
leased. Figure 4.18 indicates a typical upset welding cycle. Note that upset welding
would not be successful for larger sections because these cannot be uniformly heated
and require extremely high-amperage current. Therefore, the process is limited to
welding wires and rods up to 3/8 inch (about 10 mm) in diameter. Also, a sound joint
can be ensured only when the two surfaces being welded together have the same cross-
sectional area as well as negligible or no eccentricity.

Flash welding. Flash welding is somewhat similar to upset welding. The equipment
for flash welding includes a low-voltage transformer (5 to 10 V), a current timing

FIGURE 4.18

A typical upset welding
cycle

Pressure
force

Solid contact

Upset metal

4.2 Welding

FIGURE 4.19

Stages in a flash
welding cycle

2 £

Localized
bridges

1 I

Flashing

Upset
metal

JjL

Flashing Upsetting

Time

device, and a mechanism to compress the two workpieces against each other. Figure
4.19 illustrates the different stages involved in a flash welding cycle. We can see that
the pressure applied at the beginning is low. Therefore, there are a limited number of
contact points that act as localized bridges for the electric current. Consequently, metal
is heated at those points when the current is switched on, and the temperature increases
with the increasing current until it exceeds the melting point of the metal. At this stage,
the molten metal is expelled from the weld zone, causing "flashing." New bridges are
formed and move quickly across the whole interface, resulting in uniform heating all
over. When the whole contact area is heated above the liquidus line, electric current is
switched off, and the pressure is suddenly increased to squeeze out the molten metal,
upset the abutted parts, and weld them together.

Flash welding is used for joining large sections, rails, chain links, tools, thin-
walled tubes, and the like. It can also be employed for welding dissimilar metals. The
advantages claimed for the process include its higher productivity and its ability to
produce high-quality welds. The only disadvantage is the loss of some metal in
flashing.

Percussion welding. In percussion welding, a method of resistance welding, a high-
intensity electric current is discharged between the parts before they are brought in
solid contact. This results in an electric arc in the gap between the two surfaces. That
electric arc lasts only for about 0.001 second and is enough to melt the surfaces to a
depth of a few thousandths of an inch. The two parts are then impacted against each
other at a high speed to obtain a sound joint. The major limitation of this process is
the cross-sectional area of the welded joint. It should not exceed 0.5 square inch
(300 mm2) in order to keep the intensity of current required at a practical level. In in-
dustry, percussion welding is limited to joining dissimilar metals that cannot be welded
otherwise.

Spot welding. Figure 4.20a illustrates the principles of operation of spot welding, a re-
sistance welding process. Electric current is switched on between the welding electrodes

4 Joining of Metals

FIGURE 4.20

Resistance spot
welding: (a) principles
of operation; (b) a
cross section through a
spot weld

Copper
electrode

CXJ

Ac power
supply

(a)

Dendrites
(featherlike-
structure)

Force

Time

-X,

Nugget

(b)

to flow through the lapped sheets (workpieces) that are held together under pressure. As
can be seen in Figure 4.20b, the metal fuses in the central area of the interface between
the two sheets and then solidifies in the form of a nugget, thus providing the weld joint.
Heat is also generated at the contact areas between the electrodes and the workpieces.
Therefore, some precautions must be taken to prevent excessive temperatures and fus-
ing of the metal at those spots. The electrodes used must possess good electrical and
thermal conductivities. They are usually hollow and are water-cooled. In addition, areas
of workpieces in contact with the electrodes must be cleaned carefully.

Spot welding is the most widely used resistance welding process in industry. Car-
bon steel sheets having a thickness up to 0. 1 25 inch (4 mm) can be successfully spot
welded. Spot-welding machines have ratings up to more than 600 kVA and use a volt-
age of 1 to 12 V obtained from a step-down transformer. Multispot machines are used,
and the process can be fully automated. Therefore, spot welding has found widespread
application in the automobile, aircraft, and electronics industries, as well as in sheet
metal work.

Seam welding. Seam welding and projection welding are modifications of spot weld-
ing. In seam welding, the lapped sheets are passed between rotating circular electrodes
through which the high-amperage current flows, as shown in Figure 4.21. Electrodes
vary in diameter from less than 2 up to 14 inches (40 to 350 mm), depending upon the
curvature of the workpieces being welded. Welding current as high as 5000 A may be
employed, and the pressing force acting upon the electrodes can go up to 6 kN (more
than half a ton). A welding speed of about 12 feet per minute (4 m/min.) is quite com-
mon. Seam welding is employed in the production of pressure-tight joints used in con-
tainers, tubes, mufflers, and the like. Advantages of this process include low cost, high

4.2 Welding

FIGURE 4.21

Principles of seam
welding

Forces

Sheet metal

Overlapping nuggets

production rates, and suitability for automation. Nevertheless, the thickness of the
sheets to be seam welded is limited to 0.125 inch (4 mm) in the case of carbon steels
and much less for more conductive alloys due to the extremely high amperage required
(0.125-inch-thick steel sheets require 19,000 A, whereas aluminum sheets having the
same thickness would require 76,000 A).

Projection welding. In projection welding, one of the workpieces is purposely pro-
vided with small projections so that current flow and heating are localized at those
spots. The projections are usually produced by die pressing, and the process calls for
the use of a special upper electrode. Figure 4.22 illustrates an arrangement of two parts
to be projection welded, as well as the resulting weld nugget. As you may expect, the
projections collapse under the externally applied force after sufficient heating, thus
yielding a well-defined, fused weld nugget. When the current is switched off, the weld
cools down and solidification takes place under the applied force. The electrode force
is then released, and the welded workpiece is removed. As is the case with spot weld-
ing, the entire projection welding process takes only a fraction of a second.

Projection welding has some advantages over conventional spot welding. For in-
stance, sheets that are too thick to be joined by spot welding can be welded using this
process. Also, the presence of grease, dirt, or oxide films on the surface of the work-
pieces has less effect on the weld quality than in the case of spot welding. A further

FIGURE 4.22

An arrangement for
projection welding two
parts

Pressure

I 7

k\\\\\\\\\\\\\\\\\\\^^,,,„..r

4zzzzJ

ffir

After welding

Pressure

4 Joining of Metals

advantage of projection welding is the accuracy of locating welds inherent in that
process.

Fusion Welding Processes

Fusion welding includes a group of processes that all produce welded joints as a result
of localized heating of the edges of the base metal above its melting temperature,
wherein coalescence is produced. A filler metal may or may not be added, and no ex-
ternal pressure is required. The welded joint is obtained after solidification of the fused
weld pool.

In order to join two different metals together by fusion welding, they must possess
some degree of mutual solubility in the solid state. In fact, metals that are completely
soluble in the solid state exhibit the highest degree of weldability. Metals with limited
solid solubility have lower weldability, and metals that are mutually insoluble in the
solid state are completely unweldable by any of the fusion welding methods. In that
case, an appropriate pressure welding technique should be employed. An alternative
solution is to employ an intermediate metal that is soluble in both base metals.

Metallurgy of fusion welding. Before surveying the different fusion welding
processes, let us discuss the metallurgy of fusion welding. Important microstructural
changes take place in and around the weld zone during and after the welding operation.
Such changes in the microstructure determine the mechanical properties of the welded
joint. Therefore, a study of the metallurgy of fusion welding is essential for good de-
sign of welded joints, as well as for the optimization of the process parameters.

During fusion welding, three zones can be identified, as shown in Figure 4.23,
which indicates a single V-weld in steel after solidification and the corresponding tem-
perature distribution during welding. In the first zone, called the fusion zone, the base
metal and deposited metal (if a filler rod is used) are brought to the molten state dur-
ing welding. Therefore, when this zone solidifies after welding, it generally has a
columnar dendritic structure with haphazardly oriented grains. In other words, the mi-
crostructure of this zone is quite similar to that of the cast metal. Nevertheless, if the
molten metal is overheated during welding, this results in an acicular structure that is
brittle, has low strength, and is referred to as the Widmanstatten structure. Also, the

FIGURE 4.23

The three zones in a
fusion-welded joint and
the temperature
distribution during
welding

1500°C

700°C

2700° F

1300°F

Fusion zone

Parent metal

Heat-affected
zones

4.2 Welding 99

chemical composition of the fusion zone may change, depending upon the kind and
amount of filler metal added.

The second zone, which is referred to as the heat-affected zone (HAZ), is that por-
tion of the base metal that has not been melted. Therefore, its chemical composition
before and after welding remains unchanged. Nevertheless, its microstructure is al-
ways altered because of the rapid heating during welding and subsequent cooling. In
fact, the HAZ is subjected to a normalizing operation during welding and may conse-
quently undergo phase transformations and precipitation reactions, depending upon the
nature (chemical composition and microstructure) of the base metal. The size of the
HAZ is dependent upon the welding method employed and the nature of the base
metal. This can be exemplified by the fact that the HAZ is 0.1 inch (2.5 mm) when au-
tomatic submerged arc welding is used, ranges from 0.2 to 0.4 inch (5 to 10 mm) for
shielded-metal arc welding, and may reach 1 inch (25 mm) in conventional gas weld-
ing. This evidently affects the microstructure of the weld, which is generally fine-
grained. The effect of these structural changes on the mechanical properties of the
weld differs for different base metals. For instance, the structural changes have negli-
gible effect on the mechanical properties of low-carbon steel, regardless of the weld-
ing method used. On the contrary, when welding high-carbon alloy steel, hardened
structures like maternsite are formed in the HAZ of the weld that result in a sharp re-
duction in the ductility of the welded joint and/or crack formation. (Remember the ef-
fect of alloying elements on the critical cooling rate in the TTT diagram that you
studied in metallurgy.)

The third zone involves the unaffected parent metal adjacent to the HAZ that is
subjected to a temperature below AC3 (a critical temperature) during welding. In this
zone, no structural changes take place unless the base metal has been subjected to plas-
tic deformation prior to welding, in which case recrystallization and grain growth
would become evident.

Arc welding. Arc welding is based on the thermal effect of an electric arc that is act-
ing as a powerful heat source to produce localized melting of the base metal. The elec-
tric arc is, in fact, a sustained electrical discharge (of electrons and ions in opposite
directions) through an ionized, gaseous path between two electrodes (i.e., the anode
and the cathode). In order to ionize the air in the gap between the electrodes so that the
electric arc can consequently be started, a certain voltage is required. (The voltage re-
quired depends upon the distance between the electrodes.) The ionization process re-
sults in the generation of electrons and positively charged ions. Next, the electrons
impact on the anode, and the positively charged ions impact on the cathode. The col-
lisions of these particles, which are accelerated by the arc voltage, transform the ki-
netic energy of the particles into thermal and luminous energy, and the temperature at
the center of the arc can reach as high as 11,000°F (6000°C). Actually, only a com-
paratively low potential difference between the electrodes is required to start the arc.
For instance, about 45 V is usually sufficient for direct current (dc) welding equipment,
and up to 60 V for an alternating current (ac) welder. Also, the voltage drops after the
arc is started, and a stable arc can then be maintained with a voltage in the range of 15
to 30 V. Generally, arc welding involves using a metal electrode rod and attaching the
other electrode to the workpiece. The electrode rod either melts during the process

100

4 Joining of Metals

(consumable electrode) and provides the necessary filler metal for the weld, or the
electrode does not melt and the filler metal is separately provided.

As just mentioned, either alternating current or direct current can be used in arc
welding, although each has its distinct advantages. While arc stability is much better
with alternating current than with direct current, the ac welding equipment is far less
expensive, more compact in size, and simpler to operate. A further advantage of ac arc
welding is the high efficiency of the transformer used, which goes up to 85 percent,
whereas the efficiency of dc welding systems usually varies between only 30 and
60 percent.

In dc arc welding, the degree to which the work is heated can be regulated by
using either straight or reversed polarity. As can be seen in Figure 4.24a and b, the
cathode is the electrode rod and the anode is the workpiece in straight polarity (DCSP),
whereas it is the other way around in reversed polarity (DCRP). When using DCSP,
more heat is concentrated at the cathode (the electrode rod) than at the anode (the
workpiece). Therefore, melting and deposition rates (of consumable electrodes) are
high, but penetration in the workpiece is shallow and narrow. Consequently, DCSP is
recommended when welding sheet metal, especially at higher welding speeds. With
DCRP, heat is concentrated at the cathode (the workpiece) and results in deeper pene-
tration for a given welding condition. It is, therefore, preferred for groove welds and
similar applications.

During the welding operation, heat is generated in the transformer as well as in
other elements of the welding circuit, resulting in a temperature rise that may cause
damage to those elements. There is, therefore, a time limitation when using the weld-
ing equipment at a given amperage. That time limitation is usually referred to as the
rated duty cycle. Consider the following numerical example. A power supply for arc
welding rated at a 150- A 40-percent duty cycle means that it can be used only 40
percent of the time when welding at 150 A. The idle or unused time is required to
allow the equipment to cool down. The percentage of duty cycles at currents other than
the rated current can be calculated using the following equation:

% duty cycle

-I

rated current V

\ load current /

x rated duty cycle

(4.2)

FIGURE 4.24

Straight and reversed
polarities in dc arc
welding: (a) dc straight
polarity (DCSP); (b) dc
reversed polarity
(DCRP)

Electrode
(cathode)

Electrode
(anode)

Workpiece
(anode)

Workpiece
(cathode)

(a)

(b)

4.2 Welding

101

Therefore, for this power supply, the percentage of the duty cycle at 100 A is as
follows:

% duty cycle at 100 A = [ — V x 40% = 90%

There are various types of arc welding. They include the following methods:

1. Shielded-metal arc welding (SMAW)

2. Carbon arc welding (CAW)

3. Flux-cored arc welding (FCAW)

4. Stud arc welding (SW)

5. Submerged arc welding (SAW)

6. Gas-metal arc welding (GMAW, usually called MIG)

7. Gas-tungsten arc welding (GTAW, usually called TIG)

8. Plasma arc welding (PAW)

In addition, there is another welding process, electroslag welding (EW), which is not
based on the phenomenon of the electric arc but, nevertheless, employs equipment
similar to that used in gas-metal arc. flux-cored arc, or submerged arc welding.

1. Shielded-metal arc welding. Shielded-metal arc welding (SMAW) is a manual arc
welding process that is sometimes referred to as stick welding. The source of heat
for welding is an electric arc maintained between a flux-covered, consumable metal
electrode and the workpiece. As can be seen in Figure 4.25, which indicates the op-
erating principles of this process, shielding of the electrode tip, weld puddle, and
weld area in the base metal is ensured through the decomposition of the flux cov-
ering. A blanket of molten slag also provides shielding for the molten-metal pool.
The filler metal is provided mainly by the metal core of the electrode rod.

Shielded-metal arc welding can be used for joining thin and thick sheets of
plain-carbon steels, low-alloy steels, and even some alloy steels and cast iron, pro-
vided that the electrode is properly selected and also that preheating and postheat-
ing treatments are performed. It is actually the most commonly used welding
process and has found widespread application in steel construction and shipbuild-
ing. Nevertheless, it is uneconomical and/or impossible to employ shielded-metal
arc welding to join some alloys, such as aluminum alloys, copper, nickel, copper-
nickel alloys, and low-melting-point alloys such as zinc, tin, and magnesium alloys.

102

4 Joining of Metals

Another clear shortcoming of the process is that welding must be stopped each time
an electrode stick is consumed to allow mounting a new one. This results in idle
time and, consequently, a drop in productivity.

The core wires of electrodes used for shielded-metal arc welding have many dif-
ferent compositions. The selection of a particular electrode material depends upon
the application for which it is going to be used and the kind of base metal to be
welded. Consumable electrodes are usually coated with flux but can also be un-
coated. The metal wire can have a diameter of up to 1 5/ 32 inch ( 1 2 mm) and a length
of about 18 inches (450 mm). Although various metals are used as wire materials, by
far the most commonly used electrode materials involve low-carbon steel (for weld-
ing carbon steels) and low-alloy steel (for welding alloy steels). Electrodes can be
bare, lightly coated, or heavily coated. The electrode covering, or coating, results in
better-quality welds as it improves arc stability, produces gas shielding to prevent
oxidation and nitrogen contamination, and also provides slag, which, in turn, re-
tards the cooling rate of the weld's fusion zone. Therefore, electrode coatings are
composed of substances that serve these purposes. Table 4.1 indicates the composi-
tion of typical electrode coatings, together with the function of each constituent.

2. Carbon arc welding. In carbon arc welding (CAW), nonconsumable electrodes
made of carbon or graphite are used. Only a dc power supply can be employed,
and the electric arc is established either between a single carbon electrode and
the workpiece (Bernardos method) or between two carbon electrodes (independent
arc method). In both cases, no shielding is provided. A filler metal may be used,
especially when welding sheets with thicknesses more than 1/8 inch (3 mm). The
carbon electrodes have diameters ranging from 3/8 to 1 inch (10 to 25 mm) and are
used with currents that range from 200 to 600 A.

TABLE 4.1

The constituents of
typical electrode
coatings and their
functions

Main Function

Constituent

Percentage

Gas generating

Starch

Cellulose

25-40%

Calcium carbonate

Slag forming

Kaolin
Titanium dioxide

Feldspar

20-40%

Asbestos

Binding

Sodium silicate
Potassium silicate

20-30%

Deoxidizing

Ferrosilicon
Aluminum

5-10%

Arc stabilizing

Potassium titanate
Titanium oxide

5-10%

Increasing deposition rate

Iron powder

0-40%

Improving weld strength

Different alloying elements

5-10%

4.2 Welding

103

Carbon arc welding is not commonly used in industry. Its application is limited
to the joining of thin sheets of nonferrous metals and to brazing.

3. Flux-cored arc welding. Flux-cored arc welding (FCAW) is an arc welding process
in which the consumable electrode takes the form of a tubular, flux-filled wire, that
is continuously fed from a spool. Shielding is usually provided by the gases evolv-
ing during the combustion and decomposition of the flux contained within the
tubular wire. The process is, therefore, sometimes called inner-shielded, or self-
shielded, arc welding. Additional shielding may be acquired through the use of an
auxiliary shielding gas, such as carbon dioxide, argon, or both. In the latter case, the
process is a combination of the conventional flux-cored arc welding and the gas-
metal arc welding methods and is referred to as dual-shielded arc welding.

Flux-cored arc welding is generally applied on a semiautomatic basis, but it can
also be fully automated. In that case, the process is normally used to weld medium-
to-thick steel plates and stainless steel sheets. Figure 4.26 illustrates the operating
principles of flux-cored arc welding.

4. Stud arc welding. Stud arc welding (SW) is a special-purpose arc welding process
by which studs are welded to flat surfaces. This facilitates fastening and handling
of the components to which studs are joined and meanwhile eliminates the drill-
ing and tapping operations that would have been required to achieve the same
goal. Only dc power supplies are employed, and the process also calls for the
use of a special welding gun that holds the stud during welding. Figure 4.27 shows
the stages involved in stud arc welding, a process that is entirely controlled by the
timer of the gun. As can be seen in the figure, shielding is accomplished through the
use of a ceramic ferrule that surrounds the end of the stud during the process. Stud
arc welding requires a low degree of welding skill, and the whole welding cycle
usually takes less than a second.

5. Submerged arc welding. Submerged arc welding (SAW) is a fairly new automatic
arc welding method in which the arc and the weld area are shielded by a blanket of
a fusible granular flux. A bare electrode is used and is continuously fed by a special
mechanism during welding. Figure 4.28 shows the operating principles of sub-
merged arc welding. As can be seen from the figure, the process is used to join flat
plates in the horizontal position only. This limitation is imposed by the nature of the
flux and the way it is fed.

As is the case with previously discussed arc welding processes, gases evolve as
a result of combustion and decomposition of the flux, due to the high temperature

FIGURE 4.26

Operating principles of
flux-cored arc welding

Solidified
slag

Electrode (tube)
Flux core

Base metal

104

4 Joining of Metals

FIGURE 4.27

Stages involved in stud
arc welding

Stud

Flux

Light
pressure

■in

IC^I

Base metal

of the arc, and form a pocket, or gas bubble, around the arc. As Figure 4.29 shows,
this gas bubble is sealed from the arc by a layer of molten flux. This isolates the arc
from the surrounding atmosphere and, therefore, ensures proper shielding.

The melting temperature of the flux must be lower than that of the base metal.
As a result, the flux always solidifies after the metal, thus forming an insulating
layer over the solidifying molten metal pool. This retards the solidification of the
fused metal and, therefore, allows the slag and nonmetallic inclusions to float off
the molten pool. The final outcome is always a weld that is free of nonmetallic in-
clusions and entrapped gases and has a homogeneous chemical composition. The
flux should also be selected to ensure proper deoxidizing of the fused metal and

FIGURE 4.28

Operating principles of
submerged arc welding

FIGURE 4.29

The mechanics of
shielding in submerged
arc welding

Hose from hopper

to supply

granulated flux

Direction of weld

Base meta

Solidified
slag

Molten
flux

Granulated
flux

Solidified
weld metal

Base metal

Gas
bubble

Molten
metal

4.2 Welding

105

should contain additives that make up for the elements burned and lost during the
welding process.

Electric currents commonly used with submerged arc welding range between
3000 and 4000 A. Consequently, the arc obtained is extremely powerful and is ca-
pable of producing a large molten-metal pool as well as achieving deeper penetra-
tion. Other advantages of this process include its high welding rate, which is five to
ten times that produced by shielded-metal arc welding, and the high quality of the
welds obtained.

6. Gas-metal arc welding. The GMAW process is commonly called metal-inert-gas
(MIG) welding. It employs an electric arc between a solid, continuous, consumable
electrode and the workpiece. As can be seen in Figure 4.30, shielding is obtained
by pumping a stream of chemically inert gas, such as argon or helium, around the
arc to prevent the surrounding atmosphere from contaminating the molten metal.
(The electrode is bare, and no flux is added.) Dry carbon dioxide can sometimes be
employed as a shielding gas, yielding fairly good results.

Gas-metal arc welding is generally a semiautomatic process. However, it can
also be applied automatically by machine. In fact, welding robots and numerically
controlled MIG welding machines have gained widespread industrial application.
The gas-metal arc welding process can be used to weld thin sheets as well as rela-
tively thick plates in all positions, and the process is particularly popular when
welding nonferrous metals such as aluminum, magnesium, and titanium alloys. The
process is also used for welding stainless steel and critical steel parts.

The penetration for gas-metal arc welding is controlled by adopting DCRP and
adjusting the current density. The higher the current density is, the greater the pen-
etration is. The kind of shielding gas used also has some effect on the penetration.
For instance, helium gives the maximum penetration; carbon dioxide, the least;
argon, intermediate penetration. Thus, it is clear that higher current densities and the

FIGURE 4.30

Operating principles of
gas-metal arc welding

Spool

for feeding

electrode wire

Inert gas cylinder
for providing
shielding gas

Base metal

106

Joining of Metals

appropriate shielding gas can be employed in welding thick plates, provided that the
edges of these plates are properly prepared.

The electrode wires used for MIG welding must possess close dimensional tol-
erances and a consistent chemical composition appropriate for the desired applica-
tion. The wire diameter varies between 0.02 and 0.125 inch (0.5 and 3 mm).
Usually, MIG wire electrodes are coated with a very thin layer of copper to protect
them during storage. The electrode wire is available in the form of a spool weigh-
ing from 2V2 to 750 pounds (1 to 350 kg). As you may expect, the selection of the
composition of the electrode wire for a given material depends upon other factors,
such as the kind of shielding gas used, the conditions of the metal being welded
(i.e., whether there is an oxide film, grease, or contaminants), and, finally, the re-
quired properties of the weldment.

Gas-tungsten arc welding. Gas-tungsten arc welding (GTAW), which is usually
called tungsten-inert-gas (TIG) welding, is an arc welding process that employs the
heat generated by an electric arc between a nonconsumable tungsten electrode and
the workpiece. Figure 4.31 illustrates the operating principles of this process. As
can be seen, a filler rod may (or may not) be fed to the arc zone. The electrode, arc,
weld puddle, and adjacent areas of the base metal are shielded by a stream of either
argon or helium to prevent any contamination from the atmosphere. TIG welding is
normally applied manually and requires a relatively high degree of welder skill. It
can also be fully automated, in which case the equipment used drives the welding
torch at a preprogrammed path and speed, adjusts the arc voltage, and starts and
stops it.

Gas-tungsten arc welding is capable of welding nonferrous and exotic metals
in all positions. The list of metals that can be readily welded by this process is
long and includes alloy steels, stainless steels, heat-resisting alloys, refractory met-
als, aluminum alloys, magnesium alloys, titanium alloys, copper and nickel alloys,
and steel coated with low-melting-point alloys. The process is recommended for

FIGURE 4.31

Operating principles of
gas-tungsten arc
welding

Solidified
weld metal

Tungsten
Torch electrode

Shielding
gas

Handle

Filler rod

The welding torch

Base metal

Electrode-protecting
cap

Tungsten
electrode

4.2 Welding

107

welding very thin sheets, as thin as 0.005 inch (about 0.125 mm), for the root and
hot pass on tubing and pipes, and wherever smooth, clean welds are required (e.g.,
in food-processing equipment). Ultrahigh-quality welds can be obtained in the nu-
clear, rocketry, and submarine industries by employing a modified version of TIG
welding that involves placing carefully selected and prepared inserts in the gap be-
tween the sections to be joined and then completely fusing the inserts together with
the edges of base metal using a TIG torch.

All three types of current supplies (i.e., ac, DCSP, and DCRP) can be used with
gas-tungsten arc welding, depending upon the metal to be welded. Thin sheets of
aluminum or magnesium alloys are best welded by using DCRP, which prevents
burn-throughs, as previously explained. Nevertheless, it is recommended that an ac
power supply be used when welding normal sheets of aluminum and magnesium.
DCSP is best suited for welding high-melting-point alloys such as alloy steels,
stainless steels, heat-resisting alloys, copper alloys, nickel alloys, and titanium. In
addition to these considerations, DCRP is also helpful in removing surface oxide
films due to its cleaning action (the impacting of ions onto the surface like a grit
blasting).

8. Plasma arc welding. Figure 4.32 is a sketch of the torch employed in plasma arc
welding (PAW). The electric arc can take either of two forms: a transferred arc that
is a constricted arc between a tungsten electrode and the workpiece or a nontrans-
ferred arc between the electrode and the constricting nozzle. The gas flowing
around the arc heats up to extremely high temperatures like 60,000°F (33,000°C)
and becomes, therefore, ionized and electrically conductive; it is then referred to as
plasma. The main shielding is obtained from the hot ionized gas emerging from the
nozzle. Additional inert-gas shielding can be used when high-quality welds are re-
quired. In fact, plasma arc welding can be employed to join almost all metals in all
positions, although it is usually applied to thinner metals. Generally, the process is
applied manually and requires some degree of welder skill; however, the process is
sometimes automated in order to increase productivity.

Electroslag welding. Electroslag welding (ESW), which was developed by the Rus-
sians, is not an arc welding process but requires the use of equipment similar to that

FIGURE 4.32

The torch employed in
plasma arc welding

Nozzle

Tungsten
electrode

^J^E^J^

Shielding
gas

Workpiece

4 Joining of Metals

used in arc welding. Although an electric arc is used to start the process, heat is con-
tinuously generated as a result of the current flow between the electrode (or electrodes)
and the base metal through a pool of molten slag (flux). As we will see later, the
molten-slag pool also serves as a protective cover for the fused-metal pool.

The electroslag welding process is shown in Figure 4.33. As can be seen in the fig-
ure, the parts to be joined are set in the vertical position, with a gap of 1/2 to l!/2
inches (12 to 37 mm) between their edges. (The gap is dependent upon the thickness
of the parts.) The welding electrode (or electrodes) and the flux are fed automatically
into the gap, and an arc is established between the electrodes and the steel backing
plate to provide the initial molten-metal and slag pools. Next, the electrical resistivity
of the molten slag continuously produces the heat necessary to fuse the flux and the
filler and the base metals. Water-cooled copper plates travel upward along the joint,
thus serving as dams and cooling the fused metal in the cavity to form the weld.

Electroslag welding is very advantageous in joining very thick parts together in a
single pass without any need for beveling the edges of those parts. Therefore, the
process is widely used in industries that fabricate beds and frames for heavy machin-
ery, drums, boilers, and the like.

Gas welding. Gas welding refers to a group of oxyfuel gas processes in which the
edges of the parts to be welded are fused together by heating them with a flame ob-
tained from the combustion of a gas (such as acetylene) in a stream of oxygen. A filler
metal is often introduced into the flame to melt and, together with the base metal, form
the weld puddle. Gas welding is usually applied manually and requires good welding
skill. Common industrial applications involve welding thin-to-medium sheets and sec-
tions of steels and nonferrous metals in all positions. Gas welding is also widely used
in repair work and in restoring cracked or broken components.

The fuel gases used for producing the flame during the different gas welding
processes include acetylene, hydrogen, natural gas (94 percent methane), petroleum
gas, and vaporized gasoline and kerosene. However, acetylene is the most commonly
used gas for gas welding because it can provide a flame temperature of about 5700°F
(3150°C). Unfortunately, acetylene is ignited at a temperature as low as 790°F (420°C)
and becomes explosive in nature at pressures exceeding 1 .75 atmospheres. Therefore,
it is stored in metal cylinders, in which it is dissolved in acetone under a pressure of

FIGURE 4.33

The electroslag welding
process

Guiding
tubes

Base metal

Molten slag
Molten metal

Base plate

4.2 Welding

109

about 19 atmospheres. For more safety, acetylene cylinders are also filled with a
porous filler (such as charcoal) in order to form a system of capillary vessels that are
then saturated with the solution of acetylene in acetone.

The oxygen required for the gas welding process is stored in steel cylinders in the
liquid state under a pressure of about 150 atmospheres. It is usually prepared in spe-
cial plants by liquefying air and then separating the oxygen from the nitrogen.

The equipment required in gas welding, as shown in Figure 4.34, includes oxygen
and acetylene cylinders, regulators, and the welding torch. The regulators serve to re-
duce the pressure of the gas in the cylinder to the desired working value and keep it
that way throughout the welding process. Thus, the proportion of the two gases is con-
trolled, which determines the characteristics of the flame. Next, the welding torch
serves to mix the oxygen and the acetylene together and discharges the mixture out at
the tip, where combustion takes place.

Depending upon the ratio of oxygen to acetylene, three types of flames can be ob-
tained: neutral, reducing, and oxidizing. Figure 4.35 is a sketch of a typical oxyacety-
lene welding flame. As can be seen, the welding flame consists of three zones: the
inner luminous cone at the tip of the torch, the reducing zone, and the oxidizing zone.

The first zone, the luminous cone, consists of partially decomposed acetylene as a
result of the following reaction:

C2H2 -> 2C + H2

The carbon particles obtained are incandescent and are responsible for the white lumi-
nescence of that brightest part of the flame. Those carbon particles are partly oxidized
in the second zone, the reducing zone, yielding carbon monoxide and a large amount
of heat that brings the temperature up to about 5400°F (3000°C). Gases like hydrogen
and carbon monoxide are capable of reducing oxides. Next, complete combustion of
those gases yields carbon dioxide and water vapor that together with the excess oxy-
gen (if any) result in the third zone, the oxidizing zone. Those gases, however, form a
shield that prevents the atmosphere from coming in contact with the molten-metal
pool.

As can be expected, the extent (as well as the appearance) of each of the zones
depends upon the type of flame (i.e., the oxygen-to-acetylene ratio). When the ratio
is about 1, the flame is neutral and distinctively has the three zones just outlined. If
the oxygen-to-acetylene ratio is less than 1, a reducing, or carbonizing, flame is ob-
tained. In this case, the luminous cone is longer than that obtained with the neutral

FIGURE 4.34

The equipment required
in gas welding

Hoses

Oxygen

110

4 Joining of Metals

FIGURE 4.35

A sketch of a typical
oxyacetylene welding
flame

flame, and the outline of the flame is not sharp. This type of flame is employed in
welding cast iron and in hard-surfacing with high-speed steel and cemented carbides.
The third type of flame, the oxidizing flame, is obtained when the oxygen-to-acety-
lene ratio is higher than 1. In this case, the luminous cone is shorter than that ob-
tained with the neutral flame, and the flame becomes light blue in color. The
oxidizing flame is employed in welding brass, bronze, and other metals that have
great affinity to hydrogen.

Another method that utilizes the heat generated as a result of the combustion of
a fuel gas is known as pressure-gas welding. As the name suggests, this is actually a
pressure welding process in which the abutting edges to be welded are heated with
an oxyacetylene flame to attain a plastic state; then, coalescence is achieved by ap-
plying the appropriately high pressure. In order to ensure uniform heating of the sec-
tions, a multiple-flame torch that surrounds the sections is used. The shape of that
torch is dependent upon the outer contour of the sections to be welded, and the torch
is usually made to oscillate along its axis. Upsetting is accomplished by a special
pressure mechanism. This method is sometimes used for joining pipeline mains, rails,
and the like.

Electron-beam welding. Electron-beam welding (EBW) was developed by Dr.
Jacques Stohr (CEA-France, the atomic energy commission) in 1957 to solve a prob-
lem in the manufacturing of fuel elements for atomic power generators. The process is
based upon the conversion of the kinetic energy of a high-velocity, intense beam of
electrons into thermal energy as the accelerated electrons impact on the joint to be
welded. The generated heat then fuses the interfacing surfaces and produces the de-
sired coalescence.

Figure 4.36 shows the basic elements and working principles of an electron-beam
welding system. The system consists of an electron-beam gun (simply an electron
emitter such as a hot filament) that is electrically placed at a negative potential with re-
spect to an anode and that together with the workpiece is earth-grounded. A focus coil
(i.e., an electromagnetic lens) is located slightly below the anode in order to bring the
electron beam into focus upon the work. This is achieved by adjusting the current of
the focus coil. Additional electromagnetic coils are provided to deflect the beam from
its neutral axis as required. Because the electrons impacting the work travel at an ultra-
high velocity, the process should be carried out in a vacuum in order to eliminate any
resistance to the traveling electrons. Pressures on the order of 10 torr (1 atmosphere =
760 torr) are commonly employed, although pressures up to almost atmospheric can be
used. Nevertheless, it must be noted that the higher the pressure is, the wider and more
dispersed the electron beam becomes, and the lower the energy density is. (Energy
density is the number of kilowatts per unit area of the spot being welded.)

4.2 Welding

111

FIGURE 4.36

The basic elements and
working principles of an
electron-beam welding
system

Filament
current
control

Electron
beam

Accelerating
~ voltage
+ control

Electron-beam welding machines can be divided into two groups: low-voltage and
high-voltage machines. Low-voltage machines are those operating at accelerating volt-
ages up to 60 kV, whereas high-voltage machines operate at voltages up to 200 kV. Al-
though each of these two types has its own merits, the main consideration should be
the beam-power density, which is, in turn, dependent upon the beam power and the
(focused) spot size. In the early days of electron-beam welding, machines were usually
built to have a rating of 7.5 kW and less. Today, a continuous-duty rating of 60 kW is
quite common, and the trend is toward still higher ratings.

There are several advantages to the electron-beam welding process. They include
the following five:

1. Because of the high intensity of the electron beam used, the welds obtained are
much narrower, and the penetration in a single pass is much greater than that ob-
tained by conventional fusion welding processes.

2. The high intensity of the electron beam can also develop and maintain a bore-
hole in the workpiece, thus yielding a parallel-sided weld with a very narrow heat-
affected zone. As a consequence, the welds produced by this method have almost
no distortion, have minimum shrinkage, and are stronger than welds produced by
conventional fusion welding processes.

3. Because parallel-sided welds are obtained by this process, there is no need for edge
preparation of the workpieces (such as V- or J-grooves). Square butt-type joints are
commonly produced by electron-beam welding.

4. High welding speeds can be obtained with this process. Speeds up to 200 inches per
minute (0.09 m/s) are common, resulting in higher productivity.

5. Because the process is usually performed in a vacuum chamber at pressures on the
order of 10 torr, the resulting weld is excellent, is metallurgically clean, and has
an extremely low level of atmospheric contamination. Therefore, electron-beam

112

4 Joining of Metals

welding is especially attractive for joining refractory metals whose properties are
detrimentally affected by even low levels of contamination.

Because of the ultrahigh quality of the joints produced by electron-beam welding,
the process has found widespread use in the atomic power, jet engine, aircraft, and
aerospace industries. Nevertheless, the time required to vacuum the chamber before
each welding operation results in reduced productivity, and, therefore, the high cost of
the electron-beam welding equipment is not easily justified. This apparently kept the
process from being applied in other industries until it was automated. Today, electron-
beam welding is becoming popular for joining automotive parts such as gear clusters,
valves, clutch plates, and transmission components.

Laser-beam welding. The term laser stands for light amplified by stimulated emission
of radiation. It is, therefore, easy to see that a laser beam is actually a controlled, in-
tense, highly collimated, and coherent beam of light. In fact, a laser beam proved to be
a unique source of high-intensity energy that can be used in fusing metals to produce
welded joints having very high strength.

Figure 4.37 shows the working principles of laser-beam welding (LBW). In this
laser system, energy is pumped into a laser medium to cause it to fluoresce. This fluo-
rescence, which has a single wavelength or color, is trapped in the laser medium (laser
tube) between two mirrors. Consequently, it is reflected back and forth in an optical
resonator path, resulting in more fluorescence, which amplifies the intensity of the
light beam. The amplified light (i.e., the laser beam) finds its way out through the
partly transparent mirror, which is called the output mirror. The laser medium can be
a solid, such as a crystal made of yttrium aluminum garnet (YAG). It can also be a gas,
such as carbon dioxide, helium, or neon. In the latter case, the pumping energy input
is usually introduced directly by electric current flow.

Let us now consider the mechanics of laser-beam welding. The energy intensity of
a laser beam is not high enough to fuse a metal such as steel or copper. Therefore, it
must be focused by a highly transparent lens at a very tiny spot, 0.01 inch (0.25 mm)
in diameter, in order to increase the intensity of energy up to a level of 10 million W
per square inch (15,500 W/mm") at the focal point. The impacting laser energy is

FIGURE 4.37

The working principles
of a laser-beam welding
system

Pumping
energy input

Laser media

Totally

reflective

mirror

Output mirror
(partially transparent)

Random

fluorescence

(losses)

4.2 Welding 113

converted into heat as it strikes the surface of a metal, causing instantaneous fusion of
the metal at the focal point. Next, a cylindrical cavity, known as a keyhole, that is full
of vaporized, ionized metallic gas is formed and is surrounded by a narrow region of
molten metal. As the beam moves relative to the workpiece, the molten metal fills be-
hind the keyhole and subsequently cools and solidifies to form the weld. It is worth
mentioning that a stream of a cooling (and shielding) gas should surround the laser
beam to protect the focusing lens from vaporized metal. Usually, argon is used for this
purpose because of its low cost, although helium is actually the best cooling gas.

In spite of the high initial capital cost required, laser-beam welding has gained
widespread industrial application because of several advantages that the process pos-
sesses. Among these advantages are the following six:

1. Based on the preceding discussion of the mechanics of laser-beam welding, we
would always expect to have a very narrow heat-affected zone with this welding
method. Consequently, the chemical, physical, and mechanical properties of the
base metal are not altered, thus eliminating the need for any postwelding heat
treatment.

2. The ultrahigh intensity of energy of the laser beam at the focal point allows metals
having high melting points (refractory metals) to be welded.

3. The process can be successfully used to weld both nonconductive as well as mag-
netic materials that are almost impossible to join even with electron-beam welding.

4. The laser beam can be focused into a chamber through highly transparent windows,
thus rendering laser-beam welding suitable for joining radioactive materials and for
welding under sterilized conditions.

5. The process can be used for welding some materials that have always been consid-
ered unweldable.

6. The process can be easily automated. Numerically controlled laser-beam welding
systems are quite common and are capable of welding along a complex contour.

Since the Apollo project, laser-beam welding has become popular in the aerospace
industry. Today, the process is mainly employed for joining exotic metals such as tita-
nium, tantalum, zirconium, columbium, and tungsten. The process is especially advan-
tageous for making miniature joints as in tiny pacemaker cans, integrated-circuit
packs, camera parts, and batteries for digital watches. Nevertheless, laser-beam weld-
ing is not recommended for joining brass, zinc, silver, gold, or galvanized steel.

Welding Defects

In fusion welding processes, considerable thermal stresses develop during heating and
subsequent cooling of the workpiece, especially with those processes that result in
large heat-affected zones. Also, metallurgical changes and structural transformations
take place in the weld puddle as well as in the heat-affected zone, and these may be
accompanied by changes in the volume. Therefore, if no precautions are taken, defects

114 4 Joining of Metals

that are damaging to the function of the weldment may be generated. It is the com-
bined duty of the manufacturing engineer, the welder, and the inspector to make sure
that all weldments are free from all kinds of defects. Following is a brief survey of the
common kinds of welding defects.

Distortion. Distortion, warping, and buckling of the welded parts are welding defects
involving deformation (which can be plastic) of the structures as a result of residual
stresses. They come as a result of restraining the free movement of some parts or mem-
bers of the welded structure. They can also result from nonuniform expansion and
shrinkage of the metal in the weld area as a consequence of uneven heating and cool-
ing. Although it is possible to predict the magnitude of the residual stresses in some
simple cases (e.g., butt welding of two plates), an analysis to predict the magnitude
of these stresses and to eliminate distortion in the common case of a welded three-
dimensional structure is extremely complicated. Nevertheless, here are some recom-
mendations and guidelines to follow to eliminate distortion:

1. Preheat the workpieces to a temperature dependent on the properties of the base
metal in order to reduce the temperature gradient.

2. Clamp the various elements (to be welded) in a specially designed rigid welding
fixture. Although no distortion occurs with this method, there are always inher-
ent internal stresses. The internal stresses can be eliminated by subsequent
stress-relieving heat treatment.

3. Sometimes, it is adequate just to tack-weld the elements securely in the right posi-
tion (relative to each other) before actual-strength welds are applied. It is also ad-
visable to start by welding the section least subject to distortion first in order to
form a rigid skeleton that contributes to the balance of assembly.

4. Create a rational design of weldments (e.g., apply braces to sections most likely to
distort).

Porosity. Porosity can take the form of elongated blowholes in the weld puddle,
which is known as wormhole porosity, or of scattered tiny spherical holes. In both
cases, porosity is due mainly to either the evolution of gases during welding or the re-
lease of gases during solidification as a result of their decreasing solubility in the so-
lidifying metal. Excess sulfur or sulfide inclusions in steels are major contributors to
porosity because they generate gases that are often entrapped in the molten metal.
Other causes of porosity include the presence of hydrogen (remember the problem
caused by hydrogen in casting), contamination of the joint, and contaminants in the
flux. Porosity can be eliminated by maintaining clean workpiece surfaces, by properly
conditioning the electrodes, by reducing welding speed, by eliminating any moisture
on workpieces, and, most importantly, by avoiding the use of a base metal containing
sulfur or electrodes with traces of hydrogen.

Cracks. Welding cracks can be divided into two main groups: fusion zone cracks
and heat-affected zone cracks. The first group includes longitudinal and transverse
cracks as well as cracks appearing at the root of the weld bead. This type of cracking

4.2 Welding

115

is sometimes called hot cracking because it occurs at elevated temperatures just
after the molten metal starts to solidify. It is especially prevalent in ferrous alloys with
high percentages of sulfur and phosphorus and in alloys having large solidification
ranges.

The second type of cracking, heat-affected zone cracks, is also called cold crack-
ing. This defect is actually due to aggravation by excessive brittleness of the heat-
affected zone that can be caused by hydrogen embrittlement or by martensite
formation as a result of rapid cooling, especially in high-carbon and alloy-steel welded
joints. (Remember the effect of alloying elements on the TTT curve; they shift it to the
right, thus decreasing the critical cooling rate.) Cold cracks can be eliminated by using
a minimum potential source of hydrogen and by controlling the cooling rate of the
welded joint to keep it at a minimum (e.g., keep joints in a furnace after welding or
embed them in sand).

The use of multiple passes in welding can sometimes eliminate the need for
prewelding or postwelding heat treatment. Each pass would provide a sort of preheat-
ing for the pass to follow. This technique is often effective in the prevention of weld
cracks.

Slag inclusions. Slag entrapment in the weld zone can occur in single-pass as well as
in multipass welds. In single-pass arc welding, slag inclusions are caused by improper
manipulation of the electrode and/or factors such as too high a viscosity of the molten
metal or too rapid solidification. Some slag pushed ahead of the arc is drawn down by
turbulence into the molten-metal pool, where it becomes entrapped in the solidifying
weld metal. In multipass welds, slag inclusions are caused by improper removal of the
slag blanket after each pass.

Lack of fusion. Lack of fusion, shown in Figure 4.38, can result from a number of
causes. These include inadequate energy input, which leads to insufficient temperature
rise; improper electrode manipulation; and failure to remove oxide films and clean the
weld area prior to welding.

Lack of penetration. Lack of penetration, shown in Figure 4.39, is due to a low en-
ergy input, the wrong polarity, or a high welding speed.

Undercutting. Undercutting, shown in Figure 4.40, is a result of a high energy input
(excessive current in arc welding), which, in turn, causes the formation of a recess. As
we know, such sharp changes in the weld contour act as stress raisers and often cause
premature failure.

FIGURE 4.38

Lack of fusion

FIGURE 4.39

Lack of penetration

■mi

116

4 Joining of Metals

FIGURE 4.40

Undercutting

FIGURE 4.41

Underfilling

Underfilling. Underfilling, shown in Figure 4.41, involves a depression in the weld
face below the surface of the adjoining base metal. More filler metal has to be added
in order to prevent this defect.

Testing and Inspection of Welds

Welds must be evaluated by being subjected to testing according to codes and specifi-
cations that are different for different countries. The various types of tests can be di-
vided into two groups: destructive and nondestructive. Destructive testing always
results in destroying the specimen (the welded joint) and rendering it unsuitable for its
design function. Destructive tests can be mechanical, metallurgical, or chemical. We
next review various destructive and nondestructive testing methods.

Visual inspection. Visual inspection involves examination of the weld by the naked
eye and checking its dimensions by employing special gages. Defects such as cracks,
porosity, undercuts, underfills, or overlaps can be revealed by this technique.

Mechanical tests. Mechanical tests are generally similar to the conventional me-
chanical tests, the difference being the shape and size of the test specimen. Tensile,
bending, impact, and hardness tests are carried out. Such tests are conducted either on
the whole welded joint or on the deposited metal only.

Metallurgical tests. Metallurgical tests involve metallurgical microstructure and
macrostructure examination of specimens. Macrostructure examination reveals the
depth of penetration, the extent of the heat-affected zone, and the weld bead shape, as
well as hidden cracks, porosity, and slag inclusions. Microstructure examination can
show the presence of nitrides, martensite, or other structures that cause metallurgically
oriented welding problems.

Chemical tests. Chemical tests are carried out to ensure that the composition of the
filler metal is identical to that specified by the manufacturing engineer. Some are crude
tests, such as spark analysis or reagent analysis; however, if accurate data are required,
chemical analysis or spectrographic testing must be carried out.

Radiographic inspection. Radiographic inspection is usually performed by employ-
ing industrial X rays. This technique can reveal hidden porosity, cracks, and slag in-
clusions. It is a nondestructive test that does not destroy the welded joint.
High-penetration X rays are sometimes also employed for inspecting weldments hav-
ing thicknesses up to l!/2 inches (37 mm).

4.2 Welding

117

Pressure test. Hydraulic (or air) pressure is applied to welded conduits that are
going to be subjected to pressure during their service lives to check their tightness and
durability.

Ultrasonic testing. Ultrasonic waves with frequencies over 20 kHz are employed to
detect various kinds of flaws in the weld, such as the presence of nonmetallic inclu-
sions, porosity, and voids. This method is reliable even for testing very thick parts.

Magnetic testing. As we know from physics, the lines of magnetic flux are distorted
in such a way as to be concentrated at the sides of a flaw or a discontinuity, as seen in
Figure 4.42a and b. This test, therefore, involves magnetizing the part and then using
fine iron-powder particles that were uniformly dispersed on the surface of the part to
reveal the concentration of the flux lines at the location of the flaw. This method is suc-
cessful in detecting superficial hair cracks and pores in ferrous metal.

Ammonia penetrant test. The ammonia penetrant test is used to detect any leakage
from welded vessels. It involves filling the vessel with a mixture of compressed air and
ammonia and then wrapping it with paper that has been impregnated in a 5 percent so-
lution of mercuric nitrate. Any formation of black spots is an indication of leakage.

Fluorescent penetrant test. The part is immersed for about half an hour in oil (or
an oil mixture) and then dipped in magnesia powder. The powder adheres at any
crack location.

Design Considerations

As soon as the decision is made to fabricate a product by welding, the next step is to
decide which welding process to use. This decision should be followed by selection of
the types of joints, by determination of the locations and distribution of the welds, and,
finally, by making the design of each joint. Following is a brief discussion of the fac-
tors to be considered in each design stage.

Selection of the joint type. We have previously discussed the various joint designs
and realized that the type of joint depends upon the thickness of the parts to be welded.
In fact, there are other factors that should also affect the process of selecting a partic-
ular type of joint. For instance, the magnitude of the load to which the joint is going

FIGURE 4.42

Magnetic testing of
welds: (a) defective
weld; (b) sound weld

(a)

(b)

118

Joining of Metals

to be subjected during its service life is one other important factor. The manner in
which the load is applied (i.e., impact, steady, or fluctuating) is another factor. Whereas
the square butt, simple- V, double-V, and simple-U butt joints are suitable only for
usual loading conditions, the double-U butt joint is recommended for all loading con-
ditions. On the other hand, the square-T joint is appropriate for carrying longitudinal
shear under steady-state conditions. When severe longitudinal or transverse loads are
anticipated, other types of joints (e.g., the single-bevel-T, the double-bevel-T, and the
double-J) have to be considered. In all cases, it is obvious that cost is the decisive fac-
tor whenever there is a choice between two types of joints that would function equally
well.

Location and distribution of welds. It has been found that the direction of the linear
dimension of the weld with respect to the direction of the applied load has an effect on
the strength of the weld. In fact, it has been theoretically and experimentally proven
that a lap weld whose linear direction is normal to the direction of the applied load, as
is shown in Figure 4.43a, is 30 percent stronger than a lap weld whose linear direction
is parallel to the direction of the applied load, as shown in Figure 4.43b. In the first
case, the maximum force F that the joint can carry without any signs of failure can be
approximated by the following equation:

F = 0.707 Xhfx Gallowable (4.3)

where: £ is the weld leg

W is the length of the weld

^allowable is the allowable tensile stress of the filler material (e.g.,
electrode)

In the second case (Figure 4.43b), the strength of the joint is based on the fact that the
throat plane of the weld is subjected to pure shear stress and is given by the following
equation:

F = 0.707 x € x W x xallowable

where: € is the weld leg

W is the length of the weld

^allowable 's the allowable shear stress of the electrode

(4.4)

FIGURE 4.43

Location and
distribution of welds:
(a) weld linear direction
normal to the applied
load; (b) weld linear
direction parallel to the
applied load

Force

(a)

(b)

4.2 Welding

119

From the theory of plasticity, assuming you adopt the same safety factor in both cases,
it is easy to prove that

^allowable — / ^allowable

= 0.565 G:,

(4.5)

On the other hand, the strength of a butt-welded joint can be given by the following
equation:

F = € x W x aa

(4.6)

where t, W, and aallowable are as previously mentioned. A product designer should,
therefore, make use of this characteristic when planning the location and distribution
of welds.

Another important point to consider is the prevention of any tendency of the
welded elements to rotate when subjected to mechanical loads. A complete force
analysis must be carried out in order to determine the proper length of each weld. Let
us now consider a practical example to see the cause and the remedy for this tendency
to rotate. Figure 4.44 shows an L angle welded to a plate. Any load applied through the
angle will pass through its center of gravity. Therefore, the resisting forces that act
through the welds will not be equal; the force closer to the center of gravity of the
angle will always be larger. Consequently, if any tendency to rotate is to be prevented,
the weld that is closer to the center of gravity must be longer than the other one. Using
simple statics, it can easily be seen that

(4.7)

It is also recommended that very long welds be avoided. It has been found that two
small welds, for example, are much more effective than a single long weld.

Joint design. In addition to the procedures and rules adopted in common design prac-
tice, there are some guidelines that apply to joint design:

FIGURE 4.44

Preventing the tendency
of the welded element
to rotate by appropriate
distribution of welds

xxxx

■><■ 7'

Angle

Center of
gravity

Center of gravity
-of the angle

120

4 Joining of Metals

FIGURE 4.45

Designs that promote
or eliminate distortion
in welding:

(a) distortion caused by
unbalanced weld;

(b) and (c) methods for
reducing distortion

(a)

(b)

1. Try to ensure accessibility to the locations where welds are to be applied.

2. Try to avoid overhead welding.

3. Consider the heating effect on the base metal during the welding operation. Balance
the welds to minimize distortion. Use short, intermittent welds. Figure 4.45a shows
distortion caused by an unbalanced weld, whereas Figure 4.45b and c shows meth-
ods for reducing that distortion.

4. Avoid crevices around welds in tanks as well as grooves (and the like) that would
allow dirt to accumulate. Failure to do so may result in corrosion in the welded
joint.

5. Do not employ welding to join steels with high hardenability.

6. Do not weld low-carbon steels to alloy steels by the conventional fusion welding
methods because they have different critical cooling rates and hence cannot be suc-
cessfully welded.

7. When employing automatic welding (e.g., submerged arc), the conventional joint
design of manual welding should be changed. Wider Vs (for butt joints) are used,
and single-pass welds replace multipass welds.

4.3 SURFACING AND HARD-FACING

Surfacing involves the application of a thin deposit on the surface of a metallic work-
piece by employing a welding method such as oxyacetylene-gas welding, shielded-
metal arc welding, or automatic arc welding. The process is carried out to increase the
strength, the hardness, and the resistance to corrosion, abrasion, or wear. For the last
reason, the process is commonly known as hard-facing.

Good hard-facing practice should be aimed at achieving a strong bond between
the deposit and the base metal and also at preventing the formation of cracks and other
defects in the deposited layer. Therefore, the deposited layer should not generally ex-
ceed 3/32 inch (2 mm) and will rarely exceed 1/4 inch (6 mm). Also, the base metal

4.4 Thermal Cutting of Metals 121

should be heated to a temperature of 500°F to 950°F (350°C to 500°C) to ensure a
good metallurgical bond and to allow the deposited layer to cool down slowly.

Hard-facing permits the use of very hard wear- and corrosion-resisting com-
pounds. The materials used in this process are complex. They involve hard com-
pounds, like carbides and borides, that serve as the wear-resisting elements, and a
tough matrix composed of air-hardening steel or iron-base alloys. Such deposited ma-
terials increase the service life of a part three- or fourfold. The process is also em-
ployed in restoring worn parts.

The process of hard-facing has found widespread application in the heavy con-
struction equipment industry, in mining, in agricultural machinery, and in the petro-
leum industry. The list of parts that are usually hard-faced is long and includes, for
example, the vulnerable surfaces of chemical-process vessels, pump liners, valve seats,
drive sprockets, ripper teeth, shovel teeth, chutes, and the edges of coal recovery
augers.

4.4 THERMAL CUTTING OF METALS

In this section, we discuss the thermal cutting of metals, specifically oxyfuel flame cut-
ting and the different arc cutting processes. Although all these processes do not, by any
means, fall under the topic of joining (the action involved is opposite to that of join-
ing), they employ the same equipment as the corresponding welding process in each
case. The thermal cutting processes are not alternatives to sawing but rather are used
for cutting thick plates, 1 to 10 inches (25 to 250 mm) thick, as well as for difficult-to-
machine materials. Thermal cutting may be manual, using a hand-operated cutting
torch (or electrode), or the cutting element can be machine driven by a numerically
controlled system or by special machines called radiographs.

Oxyfuel Cutting

Oxyfuel cutting (OFC) is similar to oxyfuel welding except that an oxidizing flame
must always be used. The process is extensively used with ferrous metal having thick-
nesses up to 10 inches (250 mm). During the process, red-hot iron, directly subjected
to the flame, is oxidized by the extra oxygen in the flame; it then burns up, leaving just
ashes or slag. Also, the stream of burning gases washes away any molten metal in the
region being cut. Generally, there is a relationship between the speed of travel of the
torch or electrode and the smoothness of the cut edge: The higher the speed of travel,
the coarser the cut edge.

Although acetylene is commonly used as a fuel in this process, other gases are
also employed, including butane, methane, propane, natural gas, and a newly devel-
oped gas with the commercial name Mapp. Hydrogen is sometimes used as a fuel, es-
pecially underwater to provide a powerful preheating flame. In this case, compressed
air is used to keep water away from the flame.

The oxyfuel cutting process can be successfully employed only when the ignition
temperature of the metal being cut is lower than its melting point. Another condition for
the successful application of the process involves ensuring that the melting points of the

122 4 Joining of Metals

formed oxides are lower than that of the base metal itself. Therefore, oxyfuel cutting is
not recommended for cast iron because its ignition temperature is higher than its melt-
ing point. The process is also not appropriate for cutting stainless steel, high-alloy
chromium and chrome-nickel alloys, and nonferrous alloys because the melting points
of the oxides of these metals are higher than the melting points of the metals themselves.

Arc Cutting

There are several processes based upon utilization of the heat generated by an electric
arc. These arc cutting processes are generally employed for cutting nonferrous metals,
medium-carbon steel, and stainless steel.

Conventional arc cutting. Conventional arc cutting is similar to shielded-metal arc
welding. It should always be remembered, however, that the electrode enters the gap
of the cut, so the coating must serve as an insulator to keep the electric arc from short-
ing out. Consequently, electrodes with coatings containing iron powder are not recom-
mended for use with this process.

Air arc cutting. Air arc cutting involves preheating the metal to be cut by an electric
arc and blowing out the resulting molten metal by a stream of compressed air. The arc-
air torch is actually a steel tube through which compressed air is blown.

Oxygen arc cutting. Oxygen arc cutting is similar to air arc cutting except that oxy-
gen is blown instead of air. The process is capable of cutting cast irons and stainless
steels with thicknesses up to 2 inches (50 mm).

Carbon arc cutting. In carbon arc cutting, a carbon or graphite electrode is used. The
process has the disadvantage of consuming that electrode quickly, especially if contin-
uous cutting is carried out.

Tungsten arc cutting. The electrode used in tungsten arc cutting is made of tungsten
and has, therefore, a service life that is far longer than that of the carbon or graphite
electrodes. Tungsten arc cutting is commonly employed for stainless steel, copper,
magnesium, and aluminum.

Air-carbon arc cutting. Air-carbon arc cutting is quite similar to carbon arc cutting,
the difference being the use of a stream of compressed air to blow the molten metal
(that has been fused by the arc) out of the kerf (groove). The process cuts almost all
metals because its mechanics involve oxidation of the metal. Its applications involve
removal of welds, removal of defective welds, and dismantling of steel structures.

Plasma arc cutting. A plasma arc is employed to cut metals in plasma arc cutting
(PAC). The temperature of the plasma jet is extremely high (ten times higher than that
obtained with oxyfuel), thus enabling high-speed cutting rates to be achieved. Also, as
a consequence, the heat-affected zone formed along the edge of the kerf is usually less
than 0.05 inch (1.3 mm). Plasma arc cutting can be used for cutting stainless steel as
well as hard-to-cut alloys. A modification of the process involves using a special noz-
zle to generate a whirlpool of water on the workpiece, thus increasing the limit on the

4.5 Brazing and Soldering

123

FIGURE 4.46

Laser-beam cutting of
sheets and plates

Focusing
lens

Material— ^^^ ^^^

Molten material

thickness of the workpiece up to 3 inches (75 mm) and meanwhile improving the qual-
ity of the cut. The only limitation on plasma arc cutting is that the workpiece must be
electrically conductive.

Laser-beam cutting. The basic principles of laser-beam cutting are similar to those of
laser-beam welding. Nevertheless, laser cutting is achieved by the pressure from a jet
of gas that is coaxial with the laser beam, as shown in Figure 4.46. The function of the
gas jet is to blow away the molten metal that has been fused by the laser beam. Laser
beams can be employed in cutting almost any material, including nonconductive poly-
mers and ceramics. Also, the process is usually automated by using computerized nu-
merical control systems to control the movements of the machine table under the laser
beam so that workpieces can be cut to any desired contour. Other advantages of the
laser-beam cutting process include the straight-edged kerfs obtained, the very narrow
heat-affected zone that results, and the elimination of the part distortion experienced
with other conventional thermal cutting processes.

RAZING AND SOLDERING

Brazing and soldering are processes employed for joining solid metal components by
heating them to the proper temperature and then introducing between them a molten
filler alloy (brazing metal or solder). The filler alloy must always have a melting point
lower than that of the base metal of the components. The filler alloy must also possess
high fluidity and wettability (i.e., be able to spread and adhere to the surface of the
base metal). As you may expect, the mechanics of brazing or soldering are different
from those of welding. A strong brazed joint is obtained only if the brazing metal can
diffuse into the base metal and form a solid solution with it. Figure 4.47a and b is a
sketch of the microstructure of two brazed joints and is aimed at clarifying the me-
chanics of brazing and soldering.

Brazing and soldering can be employed to join carbon and alloy steels, nonfer-
rous alloys, and dissimilar metals. The parts to be joined together must be carefully
cleaned, degreased, and clamped. Appropriate flux is applied to remove any re-
maining oxide and to prevent any further oxidation of the metals. It is only under

124

4 Joining of Metals

FIGURE 4.47

A sketch of the
micro-structure of two
brazed joints: (a) gold
base metal; (b) low-
carbon steel base
metal

Au-Cu
solder

Diffusion
along grain
boundaries

(a)

(b)

such conditions that the filler metal can form a strong metallic bond with the base
metal.

The main difference between soldering and brazing is the melting point of the
filler metal in each case. Soft solders used in soldering have melting points below
930°F (500°C) and produce joints with relatively low mechanical strength, whereas
hard solders (brazing metals) have higher melting points, up to 1650°F (900°C), and
produce joints with high mechanical strength.

Soft solders are low-melting-point eutectic alloys. They are basically tin, lead,
cadmium, bismuth, or zinc alloys. On the other hand, brazing filler metals are alloys
consisting mainly of copper, silver, aluminum, magnesium, or nickel. Table 4.2 gives
the recommended filler alloys for different base metals. The chemical composition
and the field of application for the commonly used soft and hard solders are given in
Table 4.3.

TABLE 4.2

Recommended filler alloys for different base metals

Filler Alloys

Silver

40% Zinc

Nickel

Base Metal

Tin-Lead

Solder

Brass

Silver Copper

Steel

* X

Stainless steel

Nickel alloys

Copper (pure)

Brass or bronze

Silver (pure)

Not allowed

Aluminum

Either Sn-Zn 10

Al-Mg 3, Al-Si 5

or Al-Si 12

* = may be used
x = best used

4.5 Brazing and Soldering

125

TABLE 4.3

Most commonly used soft and hard brazing filler metals

Approximate Chemical

Brazing or Soldering

Type

Composition

Temperature

Application

Tin solder

Sn-Pb 70

360-490°F (183-255°C)

General-purpose solder

Sn-Pb 50

360-420°F (183-216°C)

Electrical application

Sn-Zn 10

509-750°F (265-400°C)

Soft soldering of
aluminum

Silver solder

Ag 25-50, Cu 20-40, Sn 0-35,

1175-1550°F (635-845°C)

Brazing of copper alloys

Cd 0-20, Zn 0-20

and silver in electronics

Brass solder

Cu-Zn 40

1670-1750°F (910-955°C)

General-purpose hard
solder

Nickel silver

Cu balance, Zn 20-30,

1720-1800° F (938-982°C)

Nickel alloys and steel

Ni 10-20

brazing

Copper

99.9% copper

2000-2100°F (1093-1150°C)

Brazing of steel

Silumin

Al-Si 12

1080-1120°F (582-605°C)

Brazing all aluminum
alloys except silumin

Fluxes

Fluxes are employed in soft soldering as well as in brazing in order to protect the
cleaned surfaces of the base metal against oxidation during those processes. In addi-
tion, fluxes enable proper wetting of the surfaces of the base metals by the molten filler
solders.

There are two kinds of fluxes for soft soldering operations: organic and inor-
ganic. The inorganic fluxes are mostly aqueous solutions of zinc and/or ammonium
chlorides. They must, however, be completely removed after the soldering operation
because of their corrosive effect. It is, therefore, completely forbidden to use inor-
ganic fluxes in soldering electronic components. On the other hand, organic fluxes do
not have such corrosive effects and are, therefore, widely used for fine soldering in
electronic circuits. The commonly used organic fluxes involve colophony, a kind of
resin with a melting point between 350°F and 390°F (180°C and 200°C), as well as
some fats.

The fluxes employed in brazing include combinations of borox, boric acid, bo-
rates, fluorides, and fluoborates together with a wetting agent. The flux can be in the
form of a liquid, slurry, powder, or paste, depending upon the brazing method used.

Soldering Techniques

The manual soldering method involves using a hand-type soldering iron that is made
of copper and has to be tinned each time before use. The iron is first heated to a tem-
perature of about 570°F (300°C), and its tip is then dipped into the flux and tinned with

126 4 Joining of Metals

the solder. Next, the iron is used for heating the prepared surfaces of the base metal
and for melting and distributing the soft solder. When the solder solidifies, it forms the
required solder seam.

Several other methods are also used for soldering. These include dip soldering and
induction soldering as well as the use of guns (blowtorches). Nevertheless, electric sol-
dering irons are still quite common.

Brazing Techniques

The selection of a preferred brazing method has to be based on the size and shape of
the joined components, the base metal of the joint, the brazing filler metal to be used,
and the production rate. When two brazing techniques are found to be equally suit-
able, cost is the deciding factor. The following brazing methods are commonly used
in industry.

Torch brazing. Torch brazing is still the most commonly used method. It is very
similar to oxyfuel flame welding in that the source of heat is a flame obtained from
the combustion of a mixture of a fuel gas (e.g., acetylene) and oxygen. The process
is very popular for repair work on cast iron and is usually applied manually, al-
though it can be used on a semiautomatic basis. In this process, however, a reduc-
ing flame should be used to heat the joint area to the appropriate brazing
temperature. A flux is then applied, and as soon as it melts, the filler metal (braz-
ing alloy) is hand-fed to the joint area. When the filler metal melts, it flows into the
clearance between the base components by capillary attraction. The filler metal
should always be melted by the heat gained by the joint and not by directly apply-
ing the flame.

Furnace brazing. Furnace brazing is performed in either a batch or a continuous con-
veyor-type furnace and is, therefore, best suited for mass production. The atmosphere
of the furnace is controlled to prevent oxidation and to suit the metals involved in the
process. That atmosphere can be dry hydrogen, dissociated ammonia, nitrogen, argon,
or any other inert gas. Vacuum furnaces are also employed, especially with brazing
materials containing titanium or aluminum. Nevertheless, a suitable flux is often em-
ployed. The filler metal must be placed in the joint before the parts go inside the fur-
nace. The filler metal can, in this case, take the form of a ring, washer, wire, powder,
or paste.

Induction brazing. In induction brazing, the components to be brazed are heated by
placing them in an alternating magnetic field that, in turn, induces an alternating cur-
rent in the components that rapidly reverses its direction. Special coils made of copper,
referred to as inductors, are employed for generating the magnetic field. The filler
metal is often placed in the joint area before brazing but can also be hand-fed by the
operator. This technique has a clear advantage, which is the possibility of obtaining a
very closely controlled heating area.

Dip brazing. Dip brazing involves dipping the joint to be brazed in a molten filler
metal. The latter is maintained in a special externally heated crucible and is covered

4.5 Brazing and Soldering 127

with a layer of flux to protect it from oxidation. Because the filler metal coats the en-
tire workpiece, this process is used only for small parts.

Salt-bath brazing. The source of heating in salt-bath brazing is a molten bath of fluo-
ride and chloride salts. The filler metal is placed in the joint area before brazing and is
also sometimes cladded. Next, the whole assembly is preheated to an appropriate tem-
perature and then dipped for 1 to 6 minutes in the salt bath. Finally, the hot brazed joint
is rinsed thoroughly in hot and cold water to remove any remaining flux or salt. Gen-
erally, this process is employed for brazing aluminum and its alloys. There is, however,
a problem associated with the process, and that is the pollution caused by the effluent
resulting from the rinsing operation.

Resistance brazing. Low-voltage, high-amperage current is used as the source of
energy in resistance brazing, as is the case with spot welding. In fact, a spot welder
can be employed to carry out this process, provided that the pressure is carefully ad-
justed so as to be just enough to secure the position of the contact where heat devel-
ops. The workpiece is held between the two electrodes, with the filler metal
preloaded at the joint area. This process is normally used for brazing of electrical
contacts and in the manufacture of copper transformer leads.

Design of Brazed Joints

For the proper design of brazed joints, two main factors have to be taken into consid-
eration. The first factor involves the mechanics of the process in that the brazing filler
metal flows through the joint by capillary attraction. The second factor is that the
strength of the filler metal is poorer than that of the base metals. The product designer
should aim for the following:

1. Ensuring that the filler metal is placed on one side of the joint and allocating a
space for locating the filler metal before (or during) the process.

2. Adjusting the joint clearance in order to ensure optimum conditions during brazing.
That clearance is dependent upon the filler metal used and normally takes a value
less than 0.005 inch (0.125 mm), except for silumin, in which case it can go up to
0.025 inch (0.625 mm).

3. Ensuring that the distance to be traveled by the filler metal is shorter than the limit
distant, as dictated by the physics of capillarity.

4. Providing enough filler metal.

5. Increasing the area of the joint because the filler metal is weaker than the base metal.

There are three types of joint-area geometries: butt, scarf, and lap. The butt joint
is the weakest, and the lap is the strongest. Nevertheless, when designing lap joints,
make sure that the joint overlap is more than 3f, where t is the thickness of the thinner
parent metal. Examples of some good and poor practices in the design of brazed joints
are shown in Figure 4.48 as guidelines for beginners in product design. Also, always
remember that brazed joints are designed to carry shear stress and not tension.

128

4 Joining of Metals

FIGURE 4.48

Good and poor
practices in the design
of brazed joints

essssxzzzzzi

Poor

sssss^zzzza

Good

ESSSZZZZZ
Better

< 3t

ggF23

Best

Poor

77t\

Good

Poor

Good

Jzzzzzzi

Poor

S KS/////A

Good

(SSS3E2ZZZ3

tsssizzzzzj IzzzzS

tzzzz

•^vq

SSSSS3

Poor

Good

TICKING OF METALS

Sticking, or adhesive bonding, of metals is becoming very popular in the automotive,
aircraft, and packaging industries because of the advantages that this technique can
offer. Thanks to the recent development in the chemistry of polymers, adhesives are
now cheap, can be applied easily and quickly, and can produce reasonably strong
joints. Adhesive bonding can also be employed in producing joints of dissimilar met-
als or combinations of metals and nonmetals like ceramics or polymers. This certainly
provides greater flexibility when designing products and eliminates the need for com-
plicated, expensive joining processes.

As we know, it is possible to stick entirely smooth metal surfaces together. It is
obvious, therefore, that the sticking action is caused by adhesive forces between the
sticking agent and the workpiece and not by the flowing and solidification of the stick-
ing agent into the pores of the workpiece as occurs, for instance, with wood. In other
words, adhesion represents attractive intermolecular forces under whose influence the
particles of a surface adhere to those of another one. There are also many opinions sup-
porting the theory that mechanical interlocking plays a role in bonding.

Adhesives

Structural adhesives are normally systems including one or more polymeric materials.
In their unhardened state (i.e., before they are applied and cured), these adhesives
can take the form of viscous liquids or solids with softening temperatures of about
212°F (100°C). The unhardened adhesive agents are often soluble in ketones, esters,
and higher alcohols, as well as in aromatic and chlorine hydrocarbons. The hardened

4.6 Sticking of Metals

129

FIGURE 4.49

The three types of
adhesive-bonded joints

adhesives, however, resist nearly all solvents. Adhesives that find industrial application
in bonding two nonmetallic workpieces include cyanacrylates, acrylics, and poly-
urethanes. Following is a brief description of the adhesives that are commonly used in
industry.

Epoxies. Epoxies are thermosetting polymers (see Chapter 8) that require the addi-
tion of a hardener or the application of heat so that they can be cured. Epoxies are con-
sidered to be the best sticking agents because of their versatility, their resistance to
solvents, and their ability to develop strong and reliable joints.

Phenolics. Phenolics are characterized by their low cost and heat resistance of up to
about 930°F (500°C). They can be cured by a hardener or by heat or can be used in
solvents that evaporate and thus allow setting to occur. Like epoxies, phenolics are
thermosetting polymers with good strength, but they generally suffer from brittleness.

Polyamide. The polyamide group of polymers is characterized by its oil and water re-
sistance. Polyamides are usually applied in the form of hot melts but can also be used
by evaporation of solvents in which they have been dissolved. Polyamides are nor-
mally used as can-seam sealants and the like. They are also used as hot-melt for shoes.

Silicones. Silicones can perform well at elevated temperatures; however, cost and
strength are the major limitations. Therefore, silicones are usually used as high-
temperature sealants.

Joint Preparation

The surfaces to be bonded must be clean and degreased because most adhesives do not
amalgamate with fats, oils, or wax. Joint preparation involves grinding with sandpaper,
grinding and filling, sand blasting, and pickling and degreasing with trichlorethylene.
Oxide films, electroplating coats, and varnish films need not be removed (as long as
they are fixed to the surface). Roughening of the surface is advantageous, provided that
it is not overdone.

Joint Design

There are basically three types of adhesive-bonded joints. They are shown in Figure
4.49 and include tension, shear, and peel joints. Most of the adhesives are weaker in
peel and tension than in shear. Therefore, when selecting an adhesive, you should al-
ways keep in mind the types of stresses to which the joint is going to be subjected. It
is also recommended that you avoid using tension and peel joints and change the de-
sign to replace these by shear joints whenever possible.

Tension

Sheer Peel

130

4 Joining of Metals

Review Questions

1. What does the riveting process involve?

2. What are rivets usually made of?

3. List some applications of riveting.

4. Spot welding could not completely replace riv-
eting. List some applications of riveting that
cannot be done by spot welding.

5. How would you define welding?

6. List five types of welded joint designs and dis-
cuss suitable applications for each type.

7. What are the types of different methods for
classifying the welding processes?

8. How would you break all the manufacturing
methods into groups according to each of
these classifying methods?

9. Explain briefly the mechanics of solid-state
welding.

10. What are the two main obstacles that must be
overcome so that successful pressure welding
can be achieved?

11. What is cold-pressure welding? Give two ex-
amples, using sketches.

12. Discuss briefly the mechanics of explosive
welding and draw a sketch to show the inter-
face between the welded parts.

13. List some industrial applications for explosive
welding.

14. Discuss briefly the mechanics of ultrasonic
welding.

15. What are the typical applications of ultrasonic
welding?

16. What are the different types of ultrasonic
welding machines? List the main components
common to all these machines.

17. What is the basic idea on which friction weld-
ing is based?

18. Explain briefly the stages involved in a fric-
tion welding operation.

19. List the various advantages claimed for fric-
tion welding.

20. List the limitations of friction welding.

21. What is the difference between friction weld-
ing and inertia welding?

22. What advantages does inertia welding have
over friction welding?

23. Give examples of some parts that are fabri-
cated by inertia welding.

24. Explain briefly the mechanics of induction
welding.

25. List some of the common industrial applica-
tions of the induction welding process.

26. What is the source of energy in thermit weld-
ing? Explain.

27. How is thermit welding performed?

28. What are the common applications of thermit
welding?

29. How does bonding take place in diffusion
bonding?

30. List the different processes that belong to re-
sistance welding.

31. Explain briefly the stages involved in a resis-
tance-butt welding process.

32. Using a sketch, explain the pressure-time and
current-time relationships in resistance-butt
welding.

33. List some of the applications of resistance-butt
welding.

34. Clarify the difference between flash welding
and butt welding.

35. Draw a graph illustrating current versus time
and pressure versus time in flash welding.

Chapter 4 Review Questions

131

36. When is flash welding recommended over butt
welding?

37. What is the major disadvantage of flash weld-
ing?

38. Explain the basic idea of percussion welding.

39. Explain briefly the mechanics of spot welding.

40. Draw a sketch of a section through a spot-
welded joint.

41. Draw a sketch to show a typical cycle for a
spot-welding machine.

42. How do you compare seam welding with spot
welding?

43. List some of the advantages of seam welding.

44. What are the industrial applications of seam
welding?

45. What is the basic idea of projection welding?

46. What are the advantages of projection weld-
ing?

47. What is the condition for two metals to be
joined together by fusion welding?

48. How many zones can be identified in a joint
produced by a conventional fusion welding
process? Discuss briefly the microstructure in
each of these zones.

49. For what do the letters HAZ stand?

50. Explain briefly the phenomenon of the electric
arc and how it can be employed in welding.

51. What are the advantages of alternating current
over direct current in arc welding?

52. What is the difference between DCSP and
DCRP? When would you recommend using
each of them?

53. What is meant by the rated duty cycle?

54. What shields the molten metal during
shielded-metal arc welding?

55. What is the main shortcoming of shielded-
metal arc welding?

56. List some of the functions of electrode coat-
ings.

57. Explain briefly the Bernardos welding method.

58. What is the main feature of the electrodes in
flux-cored arc welding?

59. What provides the shielding in flux-cored arc
welding?

60. What is stud arc welding? How is it per-
formed? List the main applications of this
process.

61. How is shielding achieved in submerged arc
welding?

62. Why must the plates to be joined by sub-
merged arc welding be horizontal only?

63. Why does submerged arc welding always
yield very high quality welds?

64. List some of the advantages of submerged arc
welding.

65. What provides shielding in MIG welding?

66. Why does the MIG welding process render it-
self suitable for automation?

67. How can the penetration for gas-metal arc
welding be controlled?

68. What is the main difference between the MIG
and the TIG welding processes?

69. List some of the applications of TIG welding.

70. In TIG welding, when would you use an ac
power supply and when would you use DCSP
and DCRP?

71. Explain briefly the mechanics and the basic
idea of plasma arc welding.

72. When is plasma arc welding most recom-
mended?

73. Do you consider electroslag welding to be a
true arc welding process?

74. How does welding take place in the elec-
troslag welding process?

75. When is electroslag welding usually recom-
mended?

76. What is the source of energy in oxyacetylene
flame welding?

132

4 Joining of Metals

77. How is acetylene stored for use in welding
operations?

78. What does the equipment required in gas
welding include? Explain the function of each
component.

79. What are the types of flames that can be ob-
tained in gas welding? How is each one ob-
tained?

80. What are the zones of a neutral flame? Discuss
the effect of the oxygen-to-acetylene ratio on
the nature of the flame obtained.

81. Explain briefly the operating principles of
electron-beam welding.

82. What are the major limitations of the electron-
beam welding process?

83. List some of the advantages of electron-beam
welding.

84. What are the major applications of electron-
beam welding?

85. For what do the letters in the word laser
stand?

86. Using a sketch, explain how a laser beam ca-
pable of carrying out welding can be gener-
ated.

87. Explain briefly the mechanics of laser-beam
welding.

88. What are the main advantages of laser-beam
welding?

89. List some of the applications of laser-beam
welding.

90. Using sketches, illustrate the commonly expe-
rienced welding defects. How can each be
avoided?

91. What are the main tests for the inspection of
welds? Discuss each briefly.

92. What are the factors affecting the selection of
the joint type?

93. On what basis are the location and distribution
of welds planned?

94. What rules would you consider when design-
ing a welded joint?

95. What is meant by hard-facing ?

96. What are the main applications of hard-fac-
ing?

97. List the main types of thermal cutting
processes. Discuss briefly the advantages and
limitations of each.

98. How do the mechanics of brazing differ from
those of welding?

99. What is the main difference between brazing
and soft soldering?

100. List some of the alloys used as brazing fillers
and mention the base metals that can be
brazed with each one.

101. List some of the commonly used soft solders.

102. What is the main function of brazing fluxes?

103. List some of the fluxes used in brazing.

104. List some of the fluxes used in soft soldering.
Discuss the limitations and applications of
each.

105. In soft soldering, how should the solder be
fused?

106. List the different brazing techniques used in
industry. Discuss the advantages and limita-
tions of each.

107. As a product designer, what factors should
you take into consideration when designing a
brazed joint?

108. In what case can sticking of metals not be re-
placed by other welding and brazing tech-
niques?

109. List some of the commonly used adhesives.
Discuss the characteristics and common appli-
cations of each.

110. What are the types of adhesive-bonded joints?
Which one is usually the strongest?

Chapter 4 Design Example

133

Problem

1. Two steel slabs, each 1/4 inch (6.35 mm) thick,
are to be joined by two fillet welds (i.e., at both
edges). If the width of each slab is 2.5 inches
(62.5 mm) and the joint is to withstand a load of
35,000 pounds (156,000 N), determine the al-
lowable tensile strength of the electrode type to
be used in welding.

2. Two steel plates, each 1/4 inch (6.35 mm) thick,
are to be joined by two fillet welds. If the joint is
to withstand a load of 50,000 pounds (222,500 N)
and an E7014 electrode (allowable tensile
strength = 21,000 lb/in.2 i.e., 145,000 KN/m2) is
used, determine the length of weld at each edge.
If the plate width is 10 inches (254 mm), how
would you distribute the weld? Draw a sketch.

3. Two steel plates, each 5/16 inch thick (7.9 mm),
are to be fillet-welded to a third one that is sand-
wiched between them. The width of each of the
first two plates is 4 inches (100 mm), whereas
the width of the third one is 6 inches (150 mm).
The two plates overlap the third one by 6 inches
(150 mm), and an E7014 electrode (allowable
tensile strength = 21,000 lb/in.2, 145,000
KN/m2) is to be used. If the joint is to withstand
a load of 190,000 pounds (846 KN), use a sketch
to illustrate a design for this joint and provide all
calculations.

An equal-leg-angle steel section 3 by 3 by 1/4
inch (75 by 75 by 6 mm) is to be welded to a
plate using an E7014 electrode (allowable tensile
strength = 21,000 lb/in.2 i.e., 145,000 KN/m2). If
the joint is to withstand a load of 10,000 pounds
(44.5 KN) coinciding with the axis of the angle,
design the joint and make a sketch indicating the
distribution of the weld to eliminate any ten-
dency of the angle to rotate.

Two mild steel pipes, each having a 3/4-inch
(19 mm) outer diameter, are to be joined to-
gether by brazing. Assuming that the joint is to
withstand an axial load of 6 tons, give a detailed
design of this joint. (Take allowable shear stress
of copper to be 6000 lb/in.2 i.e., 41,430 KN/m2.)

Two mild steel sheets, each 3/32 inch (2.4 mm)
thick, are to be brazed together using copper as a
filler material. Calculate the strength of the joint
when it is manufactured according to each of the
designs given in Figure 4.48. Compare the re-
sults and recommend the design that gives max-
imum strength.

A power supply for arc welding is rated at a
150-A 30-percent duty cycle. What will be the
percentage of actual time utilized in welding to
the total time the power supply is on if the cur-
rent employed in welding is only 125 A?

Design

r- W&4TA
Example

PROBLEM

You are required to design a flat-belt pulley so that it can be fabricated by welding.
The pulley is to be mounted on a shaft that is 1 lA inches (3 1 mm) in diameter, and the
outside diameter of the rim is 10 inches (250 mm). The rim of the pulley is to provide

134

4 Joining of Metals

a surface to transmit a torque of 600 lb ft (816 Nm) from a 2-inch- wide (50-mm) flat
belt to the shaft. The number of pulleys required is only 5.

Solution

It is advisable to start by gathering information about guidelines for the construc-
tional features of flat-belt pulleys (e.g., width of rim for a certain belt width and thick-
ness of rim). Information about the safe speeds of various sizes of pulleys should also
be collected.

Key. The best strategy is to design the key so that it will be the weak link in the
pulley-key-shaft assembly because it is easy to replace. A suitable key material is
AISI 1020 CD steel, which is commercially available as a key stock material. It has
the following mechanical properties:

Ultimate Tensile Strength (UTS) = 78,000 lb/in.2

yield stress = 66,000 lb/in."

yield stress in shear = 38,000 lb/in.2

Consider Figure 4.50. The force acting on the key is given by

„ T 600 lb ft x 12 in/ft ,, c„n

P = — = 1 1 ,520 pounds

r 0.625 inch

Take the key cross section to be 1/4 by 1/4 inch (6 by 6 mm), and its length € in inches:

. . 11,520 pounds .
shear stress in the key = < xaii0wabie

'/4X€

Take a safety factor of 2 for the key:
38,000 , _ „„„ ,. .. 2

Xallowable = ~ ~ = 19,000 lb/in.~

FIGURE 4.50

Forces acting on the
key

0. 25 inch

fl= 0.625 inch

Chapter 4 Design Example 135

Therefore,
11,520

= 19.000

!/4x€
and

£ = 2.4 inches (60 mm)
But,

11,520 ^ „ ui ♦

bearing stress = < allowable compressive stress

!/4 xVixi

< 66,000

= 33,000 lb/in.2 (safety factor of 2)
Therefore,

€ = 2.8 inches (70 mm)

We take this value to ensure safety against both shearing and compressing loads. We
should, however, round it, so the length of the key is to be 3 inches (75 mm).

Hub. Use a round seamless tube having a 2.25-inch outer diameter and 9/16-inch
wall. A suitable material is AISI 1020 CD steel. Again, the length of the hub must not
be less than 2.8 inches to keep the bearing stress below the allowable value. Take it as
2.875 inches.

Rim. Use a round seamless tube having a 10-inch outer diameter and 1/4-inch wall.
Again, a suitable material is AISI 1020 CD steel because of its availability and ability
to withstand the rubbing effect of the moving belt.

Spokes. The positioning and welding of four or five spokes would create a serious
problem and necessitates the use of a complicated welding fixture. Therefore, the
spokes are to be replaced in the design by a web. Use a 5/16-inch flat plate, machined
to have an outer diameter of 9.43 inches and an inner diameter 2.31 inches. An appro-
priate material is AISI 1020 HR steel. Because weight can be a factor, it is good prac-
tice to provide six equally spaced holes in the web by machining. These can also serve
as an aid in the handling and positioning of the web during welding.

Welding. Use conventional arc welding; an E7014 electrode (allowable tensile
stress = 21,000 lb/in.2) can be used. A fillet weld with a leg of 1/4 inch is adopted. The
force is given by

p = torque = 7200 = ^ pounds
radius 1.125 inches

H £

"' o

■ .c

t «

UJ |

O to

— ■£

LL Q

136

Chapter 4 Design Projects 137

The required length of the weld is
6400

lAx 0.707x0.57x21,000

= 3.03 inches (75 mm)

Use 2.00 inches (50 mm) of weld on each side of the hub.

The circumference of the hub equals n times 2.25, or 7.06 inches. Space four
welds, each 0.5 inch (12.5 mm) in length, equally, 90° apart around the circumference
of the hub. Welds on both sides of the web should be staggered. Adopt the same welds
at the rim. They should be safe because the shearing force is much lower (the radius is
larger than that of the hub).

Once all the dimensions and details are known, we are in a position to construct
the pulley as shown in the workshop drawing in Figure 4.51.

jgn Projects

1. Design a table for the machine shop. The table should be 4 feet (1200 mm) in
height, with a surface area of 3 by 3 feet (900 by 900 mm), and should be able to
carry a load of half a ton. Because only two tables are required, the design should
involve the use of steel angles and a plate that are to be joined together by welding.

2. Design a tank for compressed air. It has a capacity of 100 cubic feet (2.837 m3) and
can withstand an internal pressure of 40 atmospheres (ata). The number of tanks re-
quired is 50, and the tanks are going to be placed in a humid environment.

3. Design a compressed-air reservoir (tank) that is to be subjected to an extremely cor-
rosive environment. The capacity of the tank is 30 cubic feet (0.85 m3), and the
maximum gage pressure is 70 ata, but the pressure is pulsating from zero to the
maximum value about once every 5 minutes. The number of tanks required is 100.

4. A straight-toothed spur-gear wheel transmits a torque of 1200 lb ft (1632 Nm) to a
2-inch-diameter (50 mm) steel shaft (AISI 1045 CD steel). The pitch diameter of
the gear is 8 inches (200 mm), its width is 3 inches (75 mm), and the base diame-
ter is 7.5 inches (187.5 mm). Make a detailed design for the gear's blank (i.e., be-
fore the teeth are cut).

5. A mobile winch (little crane) can be moved on casters. It has a capacity of lifting
1 ton for 3 feet (0.9 m) about ground. The lifting arm can be extended, and the
winch can then lift 1/2 ton for up to 6 feet (1.8 m). Knowing that the production
volume is 4000 units and that casters and hydraulic pressure cylinders are to be pur-
chased from vendors, provide a detailed design and include full specifications of
the parts to be purchased.

138 4 Joining of Metals

6. The lifting arm for a crane is 60 feet (about 20 m), and its lifting capacity is 1 ton.
It is to be used in construction work and to be subjected to humidity, dirt, and so
on. Provide a detailed design for this arm using steel angles that are to be welded
together.

7. Design a frame for a hydraulic press for fabrication by welding. The height of the
cross arm is 12 feet (about 4 m). The cross arm is mounted (by welding) on two
vertical columns that are, in turn, welded to the base. The press can produce a max-
imum load of 200 tons by means of a hydraulic cylinder attached to the cross arm
(below it), and the stroke is 12 inches (300 mm).

TIP: The energy absorbed when the frame deforms should not exceed 2 per-
cent of the total energy output of the press.

Chapter 5

Metal Forming

-+■

INTRODUCTION

Metal forming processes have gained significant attention since World War II as
a result of the rapid increase in the cost of raw materials. Whereas machining
processes involve the removal of portions of the stock material (in the form of
chips) in order to achieve the required final shape, metal forming processes
are based upon the plastic deformation and flow of the billet material in its
solid state so as to take the desired shape. Consequently, metal forming
processes render themselves more efficient with respect to raw material uti-
lization than machining processes, which always result in an appreciable ma-
terial waste.

In fact, although metal forming techniques were employed in manufacturing
only semifinished products (like sheets, slabs, and rods) in the past, finished
products that require no further machining can be produced today by these
techniques. This was brought about by the recent developments in working
methods, as well as by the construction features of the forming machines em-
ployed. Among the advantages of these up-to-date forming techniques are high
productivity and very low material waste. Therefore, more designers tend to
modify the construction of the products manufactured by other processes to
use forming. Also, bearing in mind that metal forming methods are still being
used for producing semifinished products, it is evident that the vast majority of
all metal products are subjected to forming, at least at one stage during their
production. This latter fact clearly manifests the importance of the metal form-
ing methods.

139

140 5 Metal Forming

Generally, metal forming involves both billet and sheet metal forming. How-
ever, it has been a well-accepted convention to divide those processes into two
main groups: bulk (or massive) forming and sheet metal working. In this chap-
ter, only bulk forming processes (e.g., forging, cold forming, and rolling) are
covered; Chapter 6 deals with the working of sheet metal.

i6>

LASTIC DEFORMATION

Factors Affecting Plastic Deformation

During any forming process, the material plastically flows while the total volume of
the workpiece remains substantially constant. However, there are some marked
changes that take place on a microscopic scale within the grains and the crystal lattice
of the metal, resulting in a corresponding change in the properties of the material. This
latter change can be explained in view of the dislocation theory, which states that the
plastic deformation and flow of metal are caused by movement and transfer of dislo-
cations (defects in the crystal lattice) through the material with the final outcome of ei-
ther piling up or annihilating them. Following are some factors that affect plastic
deformation by influencing the course of dislocations.

Impurities and alloying additives. It is well known that pure metals possess higher
plasticity than their alloys. The reason is that the presence of structural components
and chemical compounds impedes the transfer and migration of dislocations, resulting
in lower plasticity.

Temperature at which deformation takes place. As a rule, the plasticity of a metal
increases with temperature, whereas its resistance to deformation decreases. The
higher the temperature, the higher the plasticity and the lower the yield point. More-
over, no work-hardening occurs at temperatures above the recrystallization tempera-
ture. This should be expected because recrystallization denotes the formation and
growth of new grains of metal from the fragments of the deformed grains, together
with restoring any distortion in the crystal lattice. Consequently, strength values drop
to the level of a nonwork-hardened state, whereas plasticity approaches that of the
metal before deformation. In fact, a forming process is termed hot if the tempera-
ture at which deformation takes place is higher than the recrystallization temperature.
Lead that is formed at room temperature in summer actually undergoes hot forming
because the recrystallization temperature for lead is 39.2°F (4°C). When deforma-
tion occurs at a temperature below the recrystallization temperature of the metal, the
process is termed cold forming. Cold forming processes are always accompanied by
work-hardening due to the piling up of dislocations. As a result, strength and hardness
increase while both ductility and notch toughness decrease. These changes can be
removed by heat treatment (annealing). On the other hand, when hot forming a
metal, the initial dendritic structure (the primary structure after casting) disintegrates
and deforms, and its crystals elongate in the direction of the metal flow. The insoluble

5.1 Plastic Deformation

141

impurities like nonmetallic inclusions (around the original grain boundaries) are drawn
and squeezed between the elongated grains. This texture of flow lines is usually re-
ferred to as the fibrous macrostructure. This fibrous macrostructure is permanent and
cannot be removed by heat treatment or further working. As a result, there is always
anisotropy of mechanical properties; strength and toughness are better in the longitu-
dinal direction of fibers. Also, during hot forming, any voids or cracks around grain
boundaries are closed, and the metal welds together, which, in turn, results in im-
provements in the mechanical properties of the metal.

Rate of deformation. It can generally be stated that the rate of deformation (strain
rate) in metal working adversely affects the plasticity of the metal (i.e., an increase in
the deformation rate is accompanied by a decrease in plasticity). Because it takes the
process of recrystallization some time to be completed, that process will not have
enough time for completion when deformation occurs at high strain rates. Therefore,
greater resistance of the metal to deformation should be expected. This does not mean
that the metal becomes brittle.

State of stress. A state of stress at a point can be simply described by the magni-
tudes and directions of the principal stresses (a stress is a force per unit area) acting on
planes that include the point in question. The state of stress is, in fact, a precise and
scientific expression for the magnitudes and the directions of the external forces acting
on the metal. All possible states of stress can be reduced to only nine main systems, as
shown in Figure 5.1. These nine cases can, in turn, be divided into three groups. The

FIGURE 5.1

The nine main systems
of the state of stress

/!'

/"l

i
? —

Uniaxial

J—

' i

.o2 Plane

»_ II., On -*-

a7 o2-

Triaxial
(solid)

142

Metal Forming

first group includes two systems that are characterized by the absence of stress (forces)
along two directions, and the stress system is therefore called uniaxial. This is the case
when stretching sheet metal having a length that considerably exceeds its width. In
each of the three systems included in the second group, it is clear that a stress along
only one of the directions is absent. Because the other two directions (stresses) form a
plane, each of these systems is referred to as a plane-stress state. It may approximately
be represented by stretching of a thin sheet in two or more directions. The remaining
group indicates the state of stress of a body, where there are stresses acting along all
three directions in space, yielding the term triaxial. In fact, most of the bulk forming
operations (forging, rolling, and wire drawing) cause states of stress that belong to this
latter group.

Load and Energy Requirement

The force required for deforming a given metal (at any unchanged desired temperature
and at usual strain-rate levels) is dependent upon the degree of deformation, which is
the absolute value of the natural logarithm of the ratio of the final length of the billet
to its original length. On the other hand, the energy consumed throughout the forming
process is equivalent to the area under the load-deformation curve for that forming
process. Therefore, that energy can be calculated if the relationship between the load
and the deformation is known. Figure 5.2 shows the degree of deformation and the en-
ergy consumed in an upsetting operation. It must be noted that in both hot and cold
working, there is an upper limit for the degree of deformation (especially in cold work-
ing) above which cracks and discontinuities in the workpiece initiate.

Preheating the Metal for Hot Forming

Before being subjected to hot forming processes, ingots (or billets) should be uni-
formly heated throughout their cross sections, without overheating or burning the
metal at the surface. This is particularly important when forming steels. Attention must
also be given to the problems of decarburization and formation of scale in order to
bring them to a minimum. The thermal gradient is another important factor that affects

FIGURE 5.2

The degree of
deformation and the
energy consumed in
upsetting

l«4

. I — . —

I Original
length

Final
length

Deformation

5.1 Plastic Deformation 143

the soundness of the deformed part. If the temperature gradient is high, thermal
stresses may initiate and can cause internal cracks. This usually happens when a por-
tion of the metal is above the critical temperature of metal (AC] or AC3) while the rest
of the billet is not. The larger the cross section of the billet and the lower its coefficient
of thermal conductivity, the steeper the temperature gradient will be, and the more li-
able to internal cracking during heating the billet becomes. In the latter case, the rate
of heating should be kept fairly low (about 2 hours per inch of section of the billet) in
order not to allow a great difference to occur between the temperatures at the surface
and the core of the billet. The metal must then be "soaked" at the maximum tempera-
ture for a period of time long enough to ensure uniformity of temperature.

The maximum temperature to which the billet is heated before forming differs
for different metals. There is usually an optimum range of temperatures within which
satisfactory forming is obtained because of increased plasticity and decreased resis-
tance to deformation. Nevertheless, any further increase in temperature above that
range may, on the contrary, result in a defective product. Burned metal and coarse
grain structure are some of the defects encountered when a metal is excessively
heated.

The ingots may be heated in soaking pits, forge hearths, chamber furnaces, or
car-bottom furnaces, which are all heated with gas. Rotary hearth furnaces represent
another type of heating furnace. In mass production or automated lines, small ob-
jects (billets) are heated using electric current and the phenomenon of induction.
This induction-heating method is quick and keeps the surfaces of the billets clean,
and temperatures can be accurately controlled. Moreover, physical equipment re-
quires limited floor space and can be fully automated.

Friction and Lubrication
in Working of Metals

Friction plays an important role in all metal forming processes and is generally con-
sidered to be undesirable because it has various harmful effects on the forming
processes, on the properties of products, and on the tool life. During the deformation
of a metal, friction occurs at the contact surface between the flowing metal and the tool
profile. Consequently, the flow of the metal is not homogeneous, which leads to the
initiation of residual stresses, with the final outcome being an unsound product with in-
ferior surface quality. Also, friction increases the pressure acting on the forming tool
(as well as the power and energy consumed) and thus results in greater wear of the
tools.

Friction in metal forming is drastically different from the conventional Columb's
friction because extremely high pressure between the mating bodies (tool and work-
piece) is involved. Recent theories on friction in metal forming indicate that it is actu-
ally the resistance to shear of a layer, where intensive shear stress is generated as a
result of relative displacement between two bodies. When these bodies have direct
metal-to-metal contact, slipping and shear flow occur in a layer adjacent to the contact
interface. But, if a surface of contact is coated with a material having low shear resis-
tance (a lubricant that can be solid or liquid), slipping takes place through that layer of

144

5 Metal Forming

lubricant and, therefore, has low resistance. This discussion indicates clearly that the
magnitude of the friction force is determined by the mechanical properties (yield point
in shear) of the layer where actual slipping occurs. Hence, it is evident that a metal
having a low yield point in shear, such as lead, can be used as a lubricant when form-
ing metals having relatively high yield strength in compression. Figure 5.3 shows the
shear layer in three different cases: solid lubrication, dry sticking friction, and hydro-
dynamic (liquid) lubrication.

In order to reduce friction and thus eliminate its harmful effects, lubricants are ap-
plied to the tool-workpiece interface in metal forming processes. The gains include
lower load and energy requirement, prevention of galling or sticking of the workpiece
metal onto the tool, better surface finish of products, and longer tool life. An important
consideration when selecting a lubricant is its activity (i.e., its ability to adhere
strongly to the surface of the metal). The activity of a lubricant can, however, be en-
hanced by adding material with high capability of adsorption, such as fat acids. Among
other factors to be considered are thermal stability, absence of poisonous fumes, and
complete burning during heat treatment of the products.

In cold forming processes, vegetable and mineral oils as well as aqueous emul-
sions are employed as lubricants. These have the advantage of acting as coolants, elim-
inating excessive heat and thus reducing the temperature of the tool. Solid polymers,
waxes, and solid soaps (sodium or calcium stearates) are also widely used in cold-
metal working.

For relatively high temperature applications, chlorinated organic compounds and
sulfur compounds are used. Solid lubricants like molybdenum disulfide and graphite
possess low-friction properties up to elevated temperatures and are, therefore, used as
solid lubricants in hot forming. Graphite is sometimes dispersed in grease, especially
in hot forging ferrous materials. Lately, use has been made of molten glass as a lubri-
cant when alloy steels and special alloys are hot formed. The glass is added in the form
of powder between the die and a hot billet. The advantages of molten glass include low
friction, excellent surface finish, and improved tool life.

FIGURE 5.3

The shear layer in three

different cases Soft

metal

Pressure

Dry sticking
friction

Liquid
lubricant

5.2 ROLLING

5.2 Rolling 145

Cold Forming Versus Hot Forming

Cold forming has its own set of advantages and disadvantages, as does hot form-
ing, and, therefore, each renders itself appropriate for a certain field of applications.
For instance, cold forming will enhance the strength of the workpiece metal, im-
prove the quality of the surface, and provide good dimensional accuracy, but the
plastic properties of the metal (elongation percentage and reduction-in-area percent-
age) and the impact strength drop. Therefore, the final properties of cold-formed
products are obtained as required by adjusting the degree of deformation and the
parameters of the postheating treatment process. Because the loads involved in cold
forming are high, this technique is generally employed in the manufacture of small
parts of soft, ductile metals, such as low-carbon steel. Also, large quantities must be
produced to justify the high cost of tooling involved. Nevertheless, if the products
are to be further processed by machining, the increased hardness caused by cold
working is a real advantage because it results in better machinability. Therefore,
cold-rolled plates and cold-drawn bars are more suitable for machining purposes
than hot-formed ones.

On the other hand, the yield strength of a metal drops significantly at elevated
temperatures, and no work-hardening occurs. Consequently, hot forming processes
are used when high degrees of deformation are required and/or when forming large
ingots or billets because the loads and energies needed are far lower than those re-
quired in cold forming. Moreover, hot forming refines the grain structure, thus pro-
ducing softer and more ductile parts suitable for further processing by cold forming
processes. However, high temperatures affect the surface quality of products, giving
oxidation and scales.

Decarburization may also occur in steels, especially when hot forming
high-carbon steel. The scales, oxides, and decarburized layers must be removed by
one or more machining processes. This slows down the production, adds machin-
ing costs, and yields waste material, resulting in lower efficiency of material uti-
lization. A further limitation of hot forming is reduced tool life due to the
softening of tool surfaces at elevated temperatures and the rubbing action of the
hot metal while flowing. This actually subjects the tools to thermal fatigue, which
shortens their life.

Hot rolling is the most widely used metal forming process because it is employed to
convert metal ingots to simple stock members called blooms and slabs. This process
refines the structure of the cast ingot, improves its mechanical properties, and elimi-
nates the hidden internal defects. The process is termed primary rolling and is fol-
lowed by further hot rolling into plates, sheets, rods, and structural shapes. Some of
these may be subjected to cold rolling to enhance their strength, obtain good surface
finish, and ensure closer dimensional tolerances. Figure 5.4 illustrates the sequence of
operations involved in manufacturing rolled products.

FIGURE 5.4

Sequence of operations involved in manufacturing rolled products

Finished products

146

5.2 Rolling

147

Principles of Rolling

The process of rolling consists of passing the metal through a gap between rolls rotat-
ing in opposite directions. That gap is smaller than the thickness of the part being
worked. Therefore, the rolls compress the metal while simultaneously shifting it for-
ward because of the friction at the roll-metal interfaces. When the workpiece com-
pletely passes through the gap between the rolls, it is considered fully worked. As a
result, the thickness of the work decreases while its length and width increase. How-
ever, the increase in width is insignificant and is usually neglected. As can be seen in
Figure 5.5, which shows the rolling of a plate, the decrease in thickness is called draft,
whereas the increase in length and the increase in width are termed absolute elonga-
tion and absolute spread, respectively. Two other terms are the relative draft and the
coefficient of elongation, which can be given as follows:

relative draft e =

Ah x 100 K- h{

x 100

(5.1)

coefficient of elongation r\ = —
But because the volume of the work is constant, it follows that

hn x bc

(5.2)

(5.3)

hf x bt

Equation 5.3 indicates that the coefficient of elongation is adversely proportional to the
ratio of the final to the original cross-sectional areas of the work.

As can be seen in Figure 5.6, the metal is deformed in the shaded area, or defor-
mation zone. The metal remains unstrained before this area and does not undergo any
further deformation after it. It can also be seen that the metal undergoing deformation
is in contact with each of the rolls along the arc AB, which is called the arc of contact.
It corresponds to a central angle, a, that is, in turn, called the angle of contact, or angle
of bite. From the geometry of the drawing and by employing simple trigonometry, it
can be shown that

cos a = 1

K - hf __ x _ Ah
2R 2R

(5.4)

FIGURE 5.5

Simple rolling of a plate

Neck

Body

Wobbler

148

5 Metal Forming

FIGURE 5.6

The deformation zone,
state of stress, and
angle of contact in
rolling

Equation 5.4 gives the relationship between the geometrical parameters of the rolling
process, the angle of contact, the draft, and the radius of the rolls. Note that in order to
ensure that the metal will be shifted by friction, the angle of contact must be less than
fi, the angle of friction, where tan P = u, (the coefficient of friction between roll surface
and metal). In fact, the maximum permissible value for the angle of contact depends
upon other factors, such as the material of the rolls, the work being rolled, and the
rolling temperature and speed. Table 5.1 indicates the recommended maximum angle
of contact for different rolling processes.

Load and Power Requirement

As can also be seen in Figure 5.6, the main stress system in the deformation zone in a
rolling process is triaxial compression, with the maximum (principal) stress acting nor-
mal to the direction of rolling. The deformed metal is exerting an equal counterforce
on each of the rolls to satisfy the equilibrium conditions. Therefore, this force normal
to the direction of rolling is important when doing the design calculations for the rolls
as well as the mill body. It is also important in determining the power consumption in

TABLE 5.1

Maximum allowable
angle of contact for
rolling

Rolling Process

Maximum Allowable
Angle of Contact

Rolling of blooms and heavy sections
Hot rolling of sheets and strips
Cold rolling of lubricated sheets

24°-30c

15°-20e

2°-10c

5.2 Rolling 149

a rolling process. Unfortunately, the exact determination of that rolling load and power
consumption is complicated and requires knowledge of theory of plasticity as well as
calculus. Nevertheless, a first approximation of the roll load can be given by the fol-
lowing simple equation:

F=Y xbx V«xM <5-5)

where Y is the average (plane-strain) yield stress assuming no spread and is equal to
\.\5Y, where Y is the mean yield stress of the metal. Therefore, Equation 5.5 should
take the form

F= l.\5YxbxVRxAh (5-6)

Equation 5.6 neglects the effect of friction at the roll-work interface and, therefore,
gives lower estimates of the load. Based on experiments carried out on a wide range
of rolling mills, this equation can be modified to account for friction by multiplying by
a factor of 1 .2. The modified equation is

F= 1.2 x 1.157x6 xV/TxA/i (5.7)

The power consumed in the process cannot be obtained easily; however, a rough esti-
mate in low-friction conditions is given by

Y xbxRx Ahxoi ,ce,

hp = _ (5.8,

where co is the angular velocity of rolls in radians per second, and Y, b, R and Ah are
all in English units.

Rolling Mills

A rolling mill includes one or more roll stands and a main drive motor, reducing gear,
stand pinion, flywheel, and coupling gear between the units. The roll stand is the main
part of the mill, where the rolling process is actually performed. It basically consists of
housings in which antifriction bearings that are used for carrying (mounting) the rolls
are fitted. Moreover, there is a screw-down mechanism to control the gap between the
rolls and thus the required thickness of the product.

Depending upon the profile of the rolled product, the body of the roll may be ei-
ther smooth for rolling sheets (plates or strips) or grooved for manufacturing shapes
such as structural members. A roll consists of a body, two necks (one on each side),
and two wobblers (see Figure 5.5). The body is the part that contacts and deforms the
metal of the workpiece. The necks rotate in bearings that act as supports, while the
wobblers serve to couple the roll to the drive. Rolls are usually made from high-
quality steel and sometimes from high-grade cast iron to withstand the very severe ser-
vice conditions to which the rolls are subjected during the rolling process, such as
combined bending and torque, friction and wear, and thermal effects. Gray cast-iron
rolls are employed in roughing passes when hot rolling steel. Cast- or forged-steel rolls
are used in blooming, slabbing, and section mills as well as in cold-rolling mills.
Forged rolls are stronger and tougher than cast rolls. Alloy-steel rolls made of chrome-
nickel or chrome-molybdenum steels are used in sheet mills.

150

5 Metal Forming

Classification of Rolling Mills

Rolling mills are classified according to the number and arrangement of the rolls in a
stand. Following are the five main types of rolling mills, as shown in Figure 5.7a
through e.

Two-high rolling mills. Two-high rolling mills, the simplest design, have a two-high
stand with two horizontal rolls. This type of mill can be nonreversing (unidirectional),
where the rolls have a constant direction of rotation, or reversing, where the rotation
and direction of metal passage can be reversed.

Three-high rolling mills. Three-high rolling mills have a three-high stand with three
rolls arranged in a single vertical plane. This type of mill has a constant direction of
rotation, and it is not required to reverse that direction.

Four-high rolling mills. In sheet rolling, the rolls should be designed as small as pos-
sible in order to reduce the rolling force F of the metal on the rolls and the power re-
quirement. If such small-diameter rolls are used alone, they will bend and result in
nonuniform thickness distribution along the width of the sheet, as shown in Figure 5.8.
For this reason, another two backup rolls are used to minimize bending and increase the
rigidity of the system. The four rolls are arranged above one another in a vertical plane.
Also, the backup rolls always have larger diameters than those of the working rolls.

Multihigh rolling mills (Sendzimir mills). Multihigh rolling mills are used particularly
in the manufacture of very thin sheets, those with a thickness down to 0.0005 inch
(0.01 mm) and a width up to 80 inches (2000 mm), into coils. In this case, the work-
ing rolls must have very small diameters (to reduce load and power consumption, as
explained before), usually in the range of 3/8 inch (10 mm) up to 1.25 inches (30 mm).

FIGURE 5.7

The five main types of
rolling mills: (a) two-
high rolling mill; (b)
three-high rolling mill;

(d) multihigh rolling mill;

(e) universal rolling mill

(a)

3!^ — ^y}.

(d)

(e)

5.2 Rolling 151

FIGURE 5.8 Original Distorted

shape roll

..— y— -y_.

Rolling thin sheets with
small-diameter rolls

Cross section of
the sheet

Such small-diameter working rolls make a drive practically impossible. They are,
therefore, driven by friction through an intermediate row of driving rolls that are, in
turn, supported by a row of backup rolls. This arrangement involves a cluster of either
12 or 20 rolls, resulting in exceptional rigidity of the whole roll system and almost
complete absence of working-roll deflections. An equivalent system that is sometimes
used is the planetary rolling mill, in which a group of small-diameter working rolls ro-
tate around a large, idle supporting roll on each side of the work.

Universal rolling mills. Universal rolling mills are used for producing blooms from in-
gots and for rolling wide-flange H beams (Gray's beams). In this type of mill, there are
vertical rolls in addition to the horizontal ones. The vertical rolls of universal mills (for
producing structural shapes) are idle and are arranged between the bearing chocks of
the horizontal rolls in the vertical plane.

The Range of Rolled Products

The range of rolled products is standardized in each country in the sense that the
shape, dimensions, tolerances, properties, and the like are given in a standard specifi-
cations handbook that differs from country to country. The whole range of rolled prod-
ucts can generally be divided into the following four groups.

Structural shapes or sections. The first group includes general-purpose sections like
round and square bars; angles; channel, H, and I beams; and special sections (with in-
tricate shapes) like rails and special shapes used in construction work and industry.
Figure 5.9 shows a variety of sections that belong to this group. These products are
rolled in either rail mills or section mills, where the body of each roll has grooves
called passes that are made in the bodies of the upper and lower rolls in such a man-
ner as to lie in the same vertical plane. They are used to impart the required shape to
the work. This process is carried out gradually (i.e., the stock is partly deformed at
each stand, or pass, in succession). The skill of a rolling engineer is to plan and con-
struct the details of a system of successive passes that ensures the adequate rolling of
blanks into the desired shape. This operation is called roll pass design. Figure 5.10a il-
lustrates the roll passes for producing rails; Figure 5.10b, those for producing an I
beam.

152

5 Metal Forming

FIGURE 5.9

Some structural shapes
or sections produced by
rolling

E^^

Square

Slab

Hexagonal

Round

bzzzzzzi

Equal-sided angle

TZZZ7ZZD

L-section

^sss

SSSSSi

I beam

VZZJVZZ2

T-section

/^»77> Channel beam ^777^77*

Rail

Plates and sheets. Plates and sheets are produced in plate and sheet mills for the hot
rolling of metal and in cold reduction mills for the production of cold-rolled coils,
where multihigh rolling mills are employed, as previously mentioned. This group of
products is classified according to thickness. A flat product with a width ranging from
5/32 inch (4 mm) up to 4 inches (100 mm) is called a plate, whereas wider and thin-
ner flat stocks are called sheets.

Special-purpose rolled shapes. This group includes special shapes, one-piece rolled
wheels, rings, balls, ribbed tubes, and die-rolled sections in which the cross section
of the bar varies periodically along its length. These kinds of bars are used in the
machine-building industry and in the construction industry for reinforcing concrete
beams and columns. Figure 5.11a shows the sequence of operations in manufacturing
a rolled wheel for railway cars; Figure 5.11b, the wheel during the final stage in the
rolling mill.

Seamless tubes. The process of manufacturing seamless tubes involves two steps:

1. Piercing an ingot or a roughened-down round blank to form a thick-walled shell

2. Rolling the obtained shell into a hollow thin-walled tube having the desired diam-
eter and wall thickness

In the first step, the solid blank is center-drilled at one end, heated to the appropriate
temperature, and then placed in the piercing mill and forced into contact with the
working rolls. There are several types of piercing mills, but the commonly used one
has barrel-shaped rolls. As Figure 5.12 shows, the axes of the two rolls are skew lines,
each deviating with a small angle from the direction of the blank axis. Also, the two
rol)s rotate in the same direction, forcing the blank to rotate and proceed against a
mandrel. A hole is formed and becomes larger; finally, a rough tube is obtained. The
milling stand is provided with side rollers for guiding the blank and the formed rough
tube during this operation. In the second step, the hollow shell (rough tube) is usually
forced over another mandrel, and the combination is longitudinally rolled at their hot

5.2 Rolling

153

FIGURE 5.10

Roll passes: (a) for producing rails; (b) for producing an I beam

Final
pass

A few
sizing
passes

THT

(b)

Final
pass

state between grooved rolls. Mills of different types are used, including continuous,
automatic, and pilger mills. Finally, a sizing operation may be performed, between siz-
ing rolls and without the use of a mandrel, at room temperature in order to improve the
properties and finish of the tubes.

Lubrication in Rolling Processes

Friction plays a very important role in a rolling process and has some beneficial ef-
fects, provided that it is not excessive. In fact, it is responsible for shifting the work
between the rolls and should not, therefore, be eliminated or reduced below an appro-
priate level. This is an important point to be taken into account when choosing a lu-
bricant for a rolling process.

154

Metal Forming

FIGURE 5.11

The production of a
railway car wheel: (a)
sequence of stages; (b)
wheel in final stage in
mill

incj^

(b)

In the cold rolling of steel, fluid lubricants of low viscosity are employed, but
paraffin is suitable for nonferrous materials like aluminum or copper alloys to avoid
staining during subsequent heat treatment. On the other hand, hot rolling is often car-
ried out without lubricants but with a flood of water to generate steam and break up
the scales that are formed. Sometimes, an emulsion of graphite or graphited grease is
used.

Defects in Rolled Products

A variety of defects in the products arise during rolling processes. A particular defect
is usually associated with a particular process and does not arise in other processes.
Following are some of the common defects in rolled products.

Edge cracking. Edge cracking occurs in rolled ingots, slabs, or plates and is believed
to be caused by either limited ductility of the work metal or uneven deformation, es-
pecially at the edges.

FIGURE 5.12

The production of
seamless tubes by
rolling

5.3 Metal Drawing 155

FIGURE 5.13

Alligatoring when rolling

aluminum slabs

Arc of \ / \

contact ~~~-V-^^ ^N- — \

Alligatoring. Figure 5.13 shows the defect of alligatoring, which is less common
than it used to be. It usually occurs in the rolling of slabs (particularly aluminum al-
loys), where the workpiece splits along a horizontal plane on exit, with the top and
bottom parts following the rotation of their respective rolls. This defect always oc-
curs when the ratio of slab thickness to the length of contact falls within the range
1.4 to 1.7.

Folds. Folds are defects occurring during plate rolling when the reduction per pass is
too small.

Laminations. Laminations associated with cracking may develop when the reduction
in thickness is excessive.

.3 METAL DRAWING

Drawing is basically a forming process that involves pulling a slender semifinished
product (like wire, bar stock, or tube) through a hole of a drawing die. The dimen-
sions of that hole are smaller than the dimensions of the original material. Metals
are usually drawn in their cold state, and the required shape may be achieved in a
single drawing operation or through several successive drawing operations, in which
case the diameters of the holes are successively decreasing. Sometimes, annealing
is carried out between the drawing operations to relieve the metal from work-
hardening. Accurate dimensions, good surface quality, increased strength and hard-
ness, and the possibility of producing very small sections are some advantages of
the drawing process. The drawing process has, therefore, wide industrial application
and is used for manufacturing thin wires, thin-walled tubes, and components with
sections that cannot be made except by machining. It is also used for sizing hot-
rolled sections.

156

Metal Forming

Preparing the Metal for Drawing

Before being subjected to the drawing process, metal blanks (wires, rods, or tubes) are
heat treated and then cleaned of scales that result from that operation. Descaling is usu-
ally done by pickling the heat-treated metal in acid solutions. Steels are pickled in ei-
ther sulfuric or hydrochloric acid or a mixture of both; copper and brass blanks are
treated in sulfuric acid, whereas nickel and its alloys are cleaned in a mixture of sul-
furic acid and potassium bichromate. After pickling, the metal is washed to remove
any traces of acid or slag from its surface. The final operation before drawing is dry-
ing the washed blanks at a temperature above 212°F (100°C). This eliminates the
moisture and a great deal of the hydrogen dissolved in the metal, thus helping to avoid
pickling brittleness.

If steel is to be subjected to several successive drawing passes, its surface should
then be conditioned for receiving and retaining the drawing lubricant. Conditioning is
performed directly after pickling and can take the form of sulling, coppering, phos-
phating, or liming. In sulling, the steel rod is given a thin coat of iron hydroxide, which
combines with lime and serves as a carrier for the lubricant. Phosphating involves ap-
plying a film of iron, manganese, or zinc phosphates to which lubricants stick very
well. Liming neutralizes the remaining acid and forms a vehicle for the lubricant. Cop-
pering is used for severe conditions and is achieved by immersing the steel rods (or
wires) in a solution of vitriol. All conditioning operations are followed by drying at a
temperature of about 650°F (300°C) in special chambers.

Wire Drawing

Drawing dies. A die is a common term for two parts: the die body and the die holder.
Die bodies are made of cemented carbides or hardened tool steel, whereas die holders
are made of good-quality tool steel that possesses high toughness. The constructional
details of a die are shown in Figure 5.14. It can be seen from the figure that the die
opening involves four zones: entry, working zone, die bearing, and exit. The entry zone
allows the lubricant to reach the working zone easily and also protects the wire (or rod)
against scoring by sharp edges. The working zone is conical in shape and has an apex

FIGURE 5.14

The constructional
details of a drawing die

Q *~ Drawing force

5.3 Metal Drawing

157

angle that ranges between 6° and 24°, depending upon the type of work and the metal
being drawn. The die bearing, sometimes called the land, is a short cylindrical zone in
which a sizing operation is performed to ensure accuracy of the shape and dimensions
of the end product. The exit zone provides back relief to avoid scoring of the drawn
wire (or rod). In a wire-drawing operation, the end of the wire is pointed by swaging
and then fed freely into the die hole so that it appears behind the die. This pointed end
is gripped by the jaws of a carriage that pull the wire through the die opening, where
it undergoes reduction in cross-sectional area and elongation in length.

Draw benches. A wire-drawing operation usually involves the use of multidie draw
benches, where the wire passes through a series of draw plates. First, the wire leaves the
coil and passes through the first drawing die. Then, it is wound two or three turns around
a capstan (drum) before it enters the next drawing die. A typical draw bench of this type
with six draw plates is shown in Figure 5.15. In practice, a bench may include from 2 up
to 22 draw plates, and the wire leaving the last die may attain a velocity of 9800 feet per
minute (50 m/s). The capstan drives are designed to provide not only forward pull after
each pass but also backward pull to the wire before it enters the next drawing die.

Lubrication. Lubrication reduces the required drawing force and the energy con-
sumed during the process, increases the service life of the die, and allows a smoother
wire surface to be obtained. Various kinds of soap are used as lubricants in wire-
drawing processes. Examples are sodium soap or calcium stearate, which is picked up
by the wire from a soap box adjacent to the die. Although they are difficult to apply
and remove, polymers are also used as solid lubricants, especially in severe conditions,
as in the case of drawing hard alloys or titanium. Various kinds of mineral and veg-
etable oils containing fatty or chlorinated additives are also used as drawing lubricants.

Mechanics of wire drawing. The state of stress during the wire-drawing process (see
Figure 5.14) involves compressive forces along two of the directions and tension along
the third one. An approximate but simple estimate of the drawing force can be given
by the following equation:

F = a{xYx£n(^

(5.9)

FIGURE 5.15

A typical multidie draw
bench

Draw plates (dies)

Two or three
j capstan drums
and draw plates

Original
wire coil

Capstan
drum

158 5 Metal Forming

where: a0 is the original area
af is the final area
Y is the mean yield stress of the metal

In Equation 5.9, the ratio ajaf is called the coefficient of elongation, or simply the
drawing ratio. In industrial practice, it is usually about 1.25 up to 1.3. Another conju-
gate term that is used in drawing processes is the reduction, given by the following
equation:

reduction r = a°~a{ x 100 (5.10)

The theoretically obtained maximum value for the reduction is 64 percent; however, it
usually does not exceed about 40 percent in industry.

Defects in wire drawing. Structural damage in the form of voids or cracks occurs in
different forms in wire-drawing processes under certain conditions. Following are
some of the defects encountered:

1. Internal bursts in wire, taking the form of repeating internal cup and cone frac-
tures (cuppy wire), usually occur when drawing heavily cold-worked copper
under conditions of light draft and very large die angles.

2. Similar centerline arrowhead fractures occur if the blank is a sheet and when the
die angle and reduction produce severe tension on the centerline.

3. Transverse surface cracking may occur as a result of longitudinal tension stresses
in the surface layers.

Tube Drawing

Diameter and thickness of pipes can be reduced by drawing. Figure 5.16 illustrates the
simplest type of tube drawing. The final tube thickness is affected by two contradict-
ing factors. The longitudinal stress tends to make the wall thinner, whereas the cir-
cumferential stress thickens it. If a large die angle is used, the thinning effect will
dominate.

The technique shown in Figure 5.17 of using a fixed plug reduces the tube diam-
eter and controls its thickness. However, a disadvantage of this type of tube drawing is
the limitation imposed on the length of the tube by the length of the mandrel. When
tubes having longer length are to be drawn, a floating mandrel like that shown in Fig-
ure 5.18 is then employed. Another method that has gained widespread application is
using a removable mandrel like that shown in Figure 5.19.

5.4 EXTRUSION

Extrusion involves forcing a billet that is enclosed in a container through an open-
ing whose cross-sectional area and dimensions are smaller than those of the original
billet. The cross section of the extruded metal will conform to that of the die open-

FIGURE 5.16

Simplest type of tube
drawing

FIGURE 5.17

Tube drawing using a
fixed plug

- — Pull

FIGURE 5.18

Tube drawing using a
floating mandrel

\\\V\V\\\\V-

^^■w

Pull

ing. Historically, extrusion was first used toward the end of the eighteenth century for
producing lead pipes. It later gained widespread industrial application for processing
nonferrous metals and alloys like copper, brass, aluminum, zinc, and magnesium. Re-
cently, with the modern developments in extrusion techniques, lubricants, and tool-
ing, other metals, such as steels, titanium, refractory metals, uranium, and thorium,
can also be extruded successfully. The stock used for extrusion is mainly a cast ingot
or a rolled billet. Any surface defects in the original billets must be removed by saw-
ing, shearing, turning, or any other appropriate machining operation before the ex-

160

Metal Forming

FIGURE 5.19

Tube drawing using a
removable mandrel

trusion process is performed. Extrusion carried out when the billets are at their cold
state is known as cold extrusion; when they are at elevated temperatures, it is known
as hot extrusion. In this latter case, the container, the die, and the pressing plunger
must be heated to a temperature of about 650°F (350°C) prior to each extrusion
cycle.

Types of Extrusion

Direct extrusion. Direct extrusion is used in the manufacture of solid and hollow
slender products and for structural shapes that cannot be obtained by any other metal
forming process. Figure 5.20 illustrates the working principles of this method, and Fig-
ure 5.21 shows the details of an extrusion die arrangement for producing channel sec-
tions. As can be seen, during an extrusion process a billet is pushed out of the die by
a plunger and then slides along the walls of the container as the operation proceeds. At
the end of the stroke, a small piece of metal (stub-end scrap) remains unextruded in the
container.

FIGURE 5.20

Principles of direct
extrusion for producing
solid objects

Plunger

Container

5.4 Extrusion

161

The extruded product is separated by shearing, and the stub-end is then ejected out
of the container after the plunger is withdrawn. Also, the leading end of the extruded
product does not undergo enough deformation. It is, therefore, poorly shaped and must
be removed as well. Obviously, the efficiency of material utilization in this case is low,
and the waste can amount to 10 or even 15 percent, as opposed to rolling, where the
waste is only 1 to 3 percent. This makes the productivity of direct extrusion quite in-
ferior to that of rolling.

FIGURE 5.21

Typical extrusion die
arrangement for
producing channel
sections

Container

162

5 Metal Forming

FIGURE 5.22

Direct extrusion for
producing hollow
objects

Figure 5.22 illustrates the technique used for producing hollow sections and tubes.
As can be seen, a mandrel or a needle passes freely through a hole in the blank and the
die opening. If the die opening is circular, an annular clearance between the die open-
ing and the mandrel results. When the metal is extruded through the annular clearance,
it forms a tube. A hole has to be pierced or drilled into the original blank before it is
extruded.

Based on this discussion, it is clear that the conventional extrusion process has the
advantages of high-dimensional accuracy and the possibility of producing complex
sections from materials having poor plasticity. On the other hand, its disadvantages in-
clude low productivity, short tool life, and expensive tooling. Therefore, the process is
usually employed for the manufacture of complex shapes with high-dimensional accu-
racy, especially when the material of the product has a low plasticity. Figures 5.23 and
5.24 show some extruded sections and parts, and Figure 5.25 shows some final prod-
ucts assembled from extruded sections.

FIGURE 5.23

Some extruded
sections (Courtesy of
Midwest Aluminum,
Inc., Kalamazoo,
Michigan)

5.4 Extrusion

163

FIGURE 5.24

Some extruded parts
(Courtesy of Midwest
Aluminum, Inc.,
Kalamazoo, Michigan)

•

Indirect extrusion. In indirect extrusion, the extrusion die is mounted on a hollow
ram that is pushed into the container. Consequently, the die applies pressure to the bil-
let, which undergoes plastic deformation. As shown in Figure 5.26, the metal flows out
of the die opening in a direction opposite to the ram motion. There is almost no slid-
ing motion between the billet and the container walls. This eliminates friction, and the
extrusion load will be lower than that required in forward direct extrusion by about 30
percent. Also, the amount of waste scrap is reduced to only 5 percent. Nevertheless, in-
direct extrusion finds only limited application due to the complexity and the cost of
tooling and press arrangement required.

Another indirect extrusion method, usually called backward or reverse extrusion,
used in manufacturing hollow sections is shown in Figure 5.27. In this case, the metal
is extruded through the gap between the ram and the container. As in indirect extrusion
for solid objects, the ram and the product travel in opposite directions.

164

5 Metal Forming

FIGURE 5.25

Some products assembled from extruded sections (Courtesy of Midwest Aluminum, Inc.
Kalamazoo, Michigan)

Hydrostatic extrusion. A radical development that eliminates the disadvantages of
cold extrusion (like higher loads) involves hydrostatic extrusion. Figure 5.28 illus-
trates the basic principles of this process, where the billet is shaped to fit the die and
surrounded by a high-pressure hydraulic fluid in a container. When the plunger is
pressed, it increases the pressure inside the container, and the resulting high pressure
forces the billet to flow through the die. Friction between the billet and the container
is thus eliminated, whereas friction between the billet and the die is markedly re-

FIGURE 5.26

Indirect extrusion for
producing solid objects

5.4 Extrusion

165

FIGURE 5.27

Indirect (backward)
extrusion for producing
hollow objects

FIGURE 5.28

Principles of
hydrostatic extrusion

Billet

Fluid

duced. Also, the buckling effect of longer billets is eliminated because virtually the
entire length of the billet is subjected to hydrostatic pressure. This makes it possible
to extrude very long billets.

Impact extrusion. Impact extrusion involves striking a cold slug of soft metal (like
aluminum) that is held in a shallow die cavity with a rapidly moving punch, thus caus-
ing the metal to flow plastically around the punch or through the die opening. The slug
itself is a closely controlled volume of metal that is lubricated and located in the die
cavity. The press is then activated, and the high-speed punch strikes the slug. A fin-
ished impacted product is extruded with each stroke of the press. These products are
not necessarily cylindrical with a circular cross section. In fact, the range of shapes
possible is very broad, including even irregular symmetrical shapes, as shown in Fig-
ures 5.29 and 5.30. There are three types of the impact extrusion processes: forward,
reverse, and combination (the names referring to the direction of motion of the de-
forming metal relative to that of the punch).

Figure 5.31 illustrates the basic principles of reverse impact extrusion. It is used
for manufacturing hollow parts with forged bases and extruded sidewalls. The flowing
metal is guided only initially; thereafter, it goes by its own inertia. This results in the
elimination of friction and, therefore, an appreciable reduction in the load and energy
required. A further advantage is the possibility of producing thinner walls.

The principles of forward impact extrusion are illustrated in Figure 5.32. It is
mainly employed in producing hollow or semihollow products with heavy flanges and
multiple diameters formed on the inside and outside. Closer wall tolerances, larger
slenderness ratios, better concentricities, and sound thinner sections are among the ad-
vantages of this process.

166

5 Metal Forming

FIGURE 5.29

Some shapes produced
by impact extrusion
(Courtesy of Metal
Impact Corporation,
Rosemont, Illinois)

Complex shapes can be produced by a combination of the two preceding
processes, which are performed simultaneously in the same single stroke, as shown in
Figure 5.33. Like the other impact extrusion methods, this process has the advantage
of cleaner product surfaces, elimination of trimming or further machining operations,
and higher strength of the parts obtained.

Mechanics of Extrusion

We can clearly see (from Figure 5.20) that an element of the deforming metal being
extruded is subjected to a state of stress involving triaxial compression. This all-around
high pressure results in a marked improvement in the plasticity of the metal. Conse-
quently, extrusion can be employed when working metal having poor plasticity, as op-
posed to rolling or wire drawing, where only ductile metals can be formed (worked).

5.4 Extrusion

167

FIGURE 5.30

Some components
produced by impacting
(Courtesy of Metal
Impact Corporation,
Rosemont, Illinois)

Load requirement. For the sake of simplicity, it is sometimes assumed that the
processes involve ideal deformation without any friction. The extrusion pressure can
then be given by the following equation:

Pextrus.on = Y X €n^ =YX (,lR

(5.11)

FIGURE 5.31

Principles of reverse
impact extrusion

168

5 Metal Forming

FIGURE 5.32

Principles of forward
impact extrusion

FIGURE 5.33

Combination impacting

where: aQ is the original cross-sectional area

af is the final cross-sectional area after extrusion

R is the extrusion ratio

Y is the mean yield stress of the metal

The extrusion load is, therefore,

F — p x a0

(5.12)

These equations are used to give only rough estimates because actual extrusion
processes involve friction and the lack of homogeneous deformation of the metal, as
will be seen later. Therefore, research workers developed several empirical formulas to
give the extrusion pressure as a function of the extrusion ratio and the mechanical
properties of the metal. A convenient formula was proposed by W. Johnson (the emi-
nent British researcher in the area of metal forming) as follows:

= Y 0.8+ l.5€n

1 -r

(5.13)

5.4 Extrusion

169

In Equation 5.13, r is the reduction given by
reduction r = — ^ — -

(5.14)

Metal flow and deformation. To study metal flow, let us consider extruding a split bil-
let involving two identical halves, with a rectangular grid engraved on the meridional
plane of each half. The separation surface is covered with lanolin or a similar appro-
priate material to prevent welding or sticking of the two halves during the process.
After extruding the split billet, the two halves are separated, and the distortion of the
grid can be investigated. Figure 5.34 shows the grid after extrusion. We can see that
the units of the grid, which were originally square in shape, became parallelograms,
trapezoids, and other shapes. The following can also be observed:

1. The velocity of the core is greater than that of the outer layers.

2. The outer layers are deformed to a larger degree than the core.

3. The leading end of the extruded part is almost undeformed.

4. The metal adjacent to the die does not flow easily, leading to the initiation of
zones where little deformation occurs. These zones are called dead-metal zones.

In fact, the preceding method for studying the metal flow is usually used with models
made of wax, plasticine, and lead to predict any defect that may occur during the ac-
tual process so that appropriate precautions can be taken in advance.

Lubrication in Extrusion

Friction at the billet-die and billet-container interfaces increases the load and the
power requirement and reduces the service life of the tooling. For these reasons, lubri-
cants are applied to the die and container walls.

As in wire drawing, soaps and various oils containing chlorinated additives or
graphite are used as lubricants in cold extrusion of most metals, whereas lanolin is usu-
ally used for the softer ones. For hot extrusion of mild steel, graphite is an adequate lu-
bricant. It is not, however, recommended for high-temperature extrusions, such as
extruding molybdenum at 3250°F (1800°C); in this case, glass is the most successful
lubricant.

Defects in Extruded Products

Defects in extruded parts usually fall into one of three main categories: surface or in-
ternal cracking, sinking (piping), and skin-inclusion defects. Cracking is caused by
secondary tensile stresses acting within a material having low plasticity. Cracking can
occur on the surface of a relatively brittle material during the extrusion process, and it

FIGURE 5.34

Distorted grid indicating
metal flow in extrusion

170

Metal Forming

may also occur in the form of fire-tree or central bursts when extruding materials like
bismuth, magnesium, 60/40 brass, steel, and brittle aluminum alloys. Piping involves
sinking of the material at the rear of the stub-end. This defect is usually encountered
toward the end of the extrusion stroke, especially when the original billets are rela-
tively short. Skin-inclusion defects may take different forms, depending upon the de-
gree of lubrication and the hardness of the surface layer of the original stock. When
extruding lubricated billets of high-copper alloys, the surface skin will slide over the
container wall and then penetrate the billet, as illustrated in Figure 5.35, where the
three different extrusion defects are sketched.

FIGURE 5.35

Three different defects
occurring in an
extrusion process

Central
burst

Piping

Skin-inclusion

Design Considerations

Conventional extrusions. When making parts that have constant cross sections, the
extrusion process is usually more economical and faster than machining, casting, or
fabricating the shapes by welding (or riveting). Also, the designer of the extruded sec-
tion is relatively free to put the metal where he or she wants. Nevertheless, there are
some design guidelines that must be taken into consideration when designing an ex-
truded section:

1. The circle size (i.e., the diameter of the smallest circle that will enclose the ex-
trusion cross section) can be as large as 31 inches (775 mm) when extruding light
metals.

2. Solid shapes are the easiest to extrude. Semihollow and hollow shapes are more
difficult to extrude, especially if they have thin walls or include abrupt changes in
wall thickness.

3. Wall thicknesses must be kept uniform. If not, all transitions must be streamlined
by generous radii at the thick-thin junctions.

4. Sharp corners at the root of a die tongue should be avoided when extruding semi-
hollow sections.

5. A complicated section should be broken into a group of simpler sections that are as-
sembled after the separate extrusion processes. In such a case, the sections should
be designed to simplify assembly; for example, they should fit, hook, or snap to-
gether. Screw slots or slots to receive other tightening material, such as plastic, may
also be provided.

5.4 Extrusion

171

Figure 5.36 illustrates some recommended designs for assembling extruded alu-
minum sections. Figure 5.37 illustrates and summarizes some recommended designs as
well as those to be avoided as general guidelines for beginning designers.

Aluminum impact extrusions. In order to accomplish good designs of aluminum im-
pact extrusions, all factors associated with and affecting the process must be taken into
account. Examples are alloy selection, tool design, lubrication, and, of course, the gen-
eral consideration of mechanical design. Following are some basic guidelines and de-
sign examples:

1. Use alloys that apply in the desired case and have the lowest strength.

2. An impact extrusion should be symmetrical around the punch.

3. Threads, cuts, projections, and the like are made by subjecting the impact extru-
sions to further processing.

4. For reverse extrusions, the ratio of maximum length to internal diameter must not
exceed 8 to avoid failure of long punches.

5. A small outer-corner radius must be provided for a reverse extrusion, but the
inner-corner radius must be kept as small as possible (see Figure 5.38a).

6. The thickness of the bottom near the wall must be 15 percent greater than the
thickness of the wall itself to prevent shear failure (see Figure 5.38b).

7. The inside bottom should not be completely flat. To avoid the possibility of the punch
skidding on the billet, only 80 percent of it at most can be flat (see Figure 5.38c).

FIGURE 5.36

Some recommended
designs for assembling
extruded aluminum
sections (Courtesy of
the Aluminum
Association, Inc.,
Washington, D.C.)

Single

Lap joints
H 2

Double

Held by

self threading

fastener

Lap-lock joints
V\ 5

Side entry Edge entry Dovetail
sliding fit

Cylindrical sliding fits
H 8 9

Fixed

Adjustable As adapted to
stair riser

172

Metal Forming

FIGURE 5.37

Some design
considerations for
conventional extrusions
(Courtesy of the
Aluminum Association,
Inc., Washington, D.C.)

Poor

\w\w

kmmmvKas

sssssssssgssgg

^^ ^^

m\\s\\\s\\s\\\w

JWs> g^^i

?//;w

•- c

ggggssas^BssaassSfl

Good

fcmmmvMs^a

^^ ^^

^mmvmw

;^\VV\VVVVVVVV\W

^^vvvv^\\\\v\v^

Reason

SYMMETRY PREFERRED IN SEMI-HOLLOW AREAS
When designing, visualize the die and tongue that will be
necessary to produce a semi-hollow shape. By keeping
the void symmetrical you lessen the chances that the die
tongue may break.

ROUNDED CORNER STRENGTHENS TONGUE
The preceding cross section has been further improved.
The die tongue is now less likely to snap off.

REDUCE AREA OF VOID-1

Further improvement results if outline can be changed to
reduce area enclosed. Reduced area means less pressure
on the tongue; easier extrusion.

AVOID HOLLOW SHAPE

Hollow and multi-hollow extruded shapes are usually
much more costly than the simple solid shape. Also less
metal has been used.

WEB GIVES BETTER DIMENSIONAL CONTROL

Metal dimensions are more easily held than gap or angle

dimensions. Web also allows thinner wall sections in this

example.

The hollow condition of the "redesigned" part can be

avoided by making the component in two pieces as shown

by the dotted line.

SMOOTH ALL TRANSITIONS

Transitions should be streamlined by a generous radius

at any thick-thin junction.

KEEP WALL THICKNESS UNIFORM

The preceding shape can be further improved by

maintaining uniform wall thickness.

In addition to using more metal, thick-thin junctions

giv rise to distortion, die breakage or surface defects

on the extrusion.

RIBS HELP STRAIGHTENING OPERATION
Wide, thin sections can be hard to straighten after
extrusion. Ribs help prevent twisting.

5.4 Extrusion

173

FIGURE 5.38

Some design
considerations for
impact extrusions: (a)
corner radii for reverse
extrusion; (b) thickness
of the bottom near the
wall; (c) inside bottom;
(d) ribs; (e) multiple-
diameter parts
(Courtesy of the
Aluminum Association,
Inc., Washington, D.C.)

Preferably small
as possible

ftfr77777f

Approx. "^
15° 3_

Ar-Jf

r- as
small as
*^ possible / J

xgzzzzzzzzZSz

Bottom = 1.151V

[*— 0.8D-*]

approx.

approx.
(c)

Good

Poor

(d)

(e)

8. External and internal bosses are permitted, provided that they are coaxial with the
part. However, the diameter of the internal boss should not be more than 1/4 of the
internal diameter of the shell.

9. Longitudinal ribs, whether external or internal, on the full length of the impact ex-
trusion are permitted. They should preferably be located in a symmetrical distrib-
ution. However, the height of each rib must not exceed double the thickness of the
wall of the shell (see Figure 5.38d). The main function of ribs is to provide stiff-
ness to the walls of shells. They are also sometimes used for other reasons, such
as to provide locations for drilling and tapping (to assemble the part), to enhance
cooling by radiation, and to provide an appropriate gripping surface.

174

Metal Forming

10. An impact extrusion can have a wall with varying thickness along its length (i.e.,
it can be a multiple-diameter part). However, internal steps near the top of the
product should be avoided because they cause excessive loading and wear of the
punch (see Figure 5.38e).

11. Remember that it is sometimes impossible to obtain the desired shape directly by
impacting. However, an impact extrusion can be considered as an intermediate prod-
uct that can be subjected to further working or secondary operations like machining,
flange upsetting, nosing and necking, or ironing (see Figure 5.39a through c).

Again, in addition to the preceding guidelines, general rules of mechanical design
as well as common engineering sense are necessary for obtaining a successful design
for the desired product. It would also be beneficial for the beginner to look at various
designs of similar parts and to consult with experienced people before starting the de-
sign process. Given in Figure 5.40 are sketches reflecting good design practice for
some impact-extruded tubular parts and shells.

FIGURE 5.39

Some secondary
operations after impact
extruding: (a) flange
upsetting; (b) nosing;
(c) ironing (Courtesy of
the Aluminum
Association, Inc.,
Washington, D.C.)

(b)

(c)

5.4 Extrusion

175

FIGURE 5.40

Sketches reflecting
good design practice
for some impact-
extruded tubular parts
and shells (Courtesy of
Aluminum Association,
Inc., Washington, D.C.)

Tubular Parts

<^yyyyyyyyyyyy^A^yyyyM^^^_

^v^^^^^^^Mw^^^^^^^^^^^^^^^^y

Flanged tube with open end.

vzzzzzzzzzzzzzz

Cup and tube assembly,
extruded as single piece.

>/SSS/SSSSS/////////////77Z.

'/ss;;;s>;;;;ss/////w/777;

Flange end closed.

W//////////////&&

V////////////////J&

Flange with multiple
step-down diameters.

Flanged tube with
multiple diameters.

Partially closed end tube
with heavy flange.

rysss;ss;y>y's;/s;;;s>;s;;sss//sS;/;,

^ssssssssssss//////;;/;.

'///////////////S^z/A

yyyyyyyyy/y/y/y///y/yA

Combination impact with the
flange at the midpoint. Such an
impact also serves as a transition
from one diameter to another.
Wall thicknesses can also be varied.

Shells

Uniform wall thickness
with flanged end open.

Outside longitudinal ribs can be
spaced equally or in symmetrical
patterns. Ribs may be extended
to become cooling fins.

Short recessed ribs in bottom can be
used for tool insertions, drive, etc.

>///>////>//>//*/?,

An external boss can be combined
with an internal center tube.

,', 1 J > > > 1 > 1 > > / S J J / l-7-TTs

r-T-j i >/>>>! ///>// m

An integral center tube can be
formed so that assembly and
machining are not needed.

Inside bosses can be produced
as integral parts of the closed
end. The side wall can have
longitudinal internal ribs.

Combination impact.

176

Metal Forming

ORGING

The term forging is used to define the plastic deformation of metals at elevated tem-
peratures into predetermined shapes using compressive forces that are exerted through
dies by means of a hammer, a press, or an upsetting machine. Like other metal form-
ing processes, forging refines the microstructure of the metal, eliminates the hidden de-
fects such as hair cracks and voids, and rearranges the fibrous macrostructure to
conform with the metal flow. It is mainly the latter factor that gives forging its merits
and advantages over casting and machining. By successful design of the dies, the
metal flow during the process can be employed to promote the alignment of the fibers
with the anticipated direction of maximum stress. A typical example is shown in Fig-
ure 5.41, which illustrates the fibrous macrostructure in two different crankshafts pro-
duced by machining from a bar stock and by forging. As can be seen, the direction
of the fibers in the second case is more favorable because the stresses in the webs
when the crankshaft is in service coincide with the direction of fibers where the
strength is maximum.

A large variety of materials can be worked by forging. These include low-carbon
steels, aluminum, magnesium, and copper alloys, as well as many of the alloy steels
and stainless steels. Each metal or alloy has its own plastic forging temperature range.
Some alloys can be forged in a wide temperature range, whereas others have narrow
ranges, depending upon the constituents and the chemical composition. Usually, the
forging temperatures recommended for nonferrous alloys and metals are much lower
than those required for ferrous materials. Table 5.2 indicates the range of forging tem-
peratures for the commonly used alloys.

Forged parts vary widely in size ranging from a few pounds (less than a kilogram)
up to 300 tons (3 MN) and can be classified into small, medium, and heavy forgings.

FIGURE 5.41

The fibrous
macrostructure in two
crankshafts produced
by machining and by
forging

Produced by machining
from a bar stock

Produced by forging

TABLE 5.2

Forging temperature
range for different
metals

Metal

Forging Temperature

Low-carbon steel

1450-2550°F (800-1400°C)

Aluminum

645-900°F (340-480°C)

Magnesium

645-800°F (340-430°C)

Copper

800-1900°F (430-1040°C)

Brass

1100-1700°F (590-930°C)

5.5 Forging 177

Small forgings are illustrated by small tools such as chisels and tools used in cutting
and carving wood. Medium forgings include railway-car axles, connecting rods, small
crankshafts, levers, and hooks. Among the heavier forgings are shafts of power-plant
generators, turbines, and ships, as well as columns of presses and rolls for rolling
mills. Small and medium forgings are forged from rolled sections (bar stocks and
slabs) and blooms, whereas heavier parts are worked from ingots.

All forging processes fall under two main types: open-die forging processes, in
which the metal is worked between two flat dies, and closed-die forging processes, in
which the metal is formed while being confined in a closed impression of a die set.

Open-Die Forging

Open-die forging is sometimes referred to as smith forging and is actually a develop-
ment or a modern version of a very old type of forging, blacksmithing, that was prac-
ticed by armor makers and crafts people. Blacksmithing required hand tools and was
carried out by striking the heated part repeatedly by a hammer on an anvil until the de-
sired shape was finally obtained. Nowadays, blacksmith forging is used only when low
production of light forgings is required, which is mainly in repair shops. Complicated
shapes having close tolerances cannot be produced economically by this process.

The modern version of blacksmithing, open-die forging, involves the substitution
of a power-actuated hammer or press for the arm, hand hammer, and anvil of the smith.
This process is used for producing heavy forgings weighing up to more than 300 tons,
as well as for producing small batches of medium forgings with irregular shapes that
cannot be produced by modern closed-die forging. The skill of the operator plays an
important role in achieving the desired shape of the part by manipulating the heated
metal during the period between successive working strokes. Accordingly, the shape
obtained is just an approximation of the required one, and subsequent machining is al-
ways used in order to produce the part that accurately conforms to the blueprint pro-
vided by the designer.

Open-die forging operations. A smith-forging process usually consists of a group of
different operations. Among the operations employed in smith forging are upsetting,
drawing out, fullering, cutting off, and piercing. The force and energy required differ
considerably from one operation to another, depending upon the degree of "confine-
ment" of the metal being worked. Following is a brief description of some of these
operations:

1. Upsetting. Upsetting involves squeezing the billet between two flat surfaces, thus
reducing its height due to the increase in the cross-sectional area. As can be seen in
Figure 5.42a, the state of stress is uniaxial compression. In practice, however, the
billets' surfaces in contact with the die are subjected to substantial friction forces
that impede the flow of the neighboring layers of metal. This finally results in a het-
erogeneous deformation and in barreling of the deformed billet. To obtain uniform
deformation, the billet-die interfaces must be adequately lubricated.

2. Drawing out. In drawing out, the workpiece is successively forged along its length
between two dies having limited width. This results in reducing the cross-sectional

178

5 Metal Forming

FIGURE 5.42

Various smith-forging
operations: (a)
upsetting; (b) drawing
out; (c) piercing a short
billet; (d) piercing a
long billet; (e) cutting
off; (f) bending

(b)

(a)

(c)

I — . — 1 L — [ — __l I _ _j _ - 1

(d)

r ><

Thinning

Upsetting

(f)

area of the workpiece while increasing its length, as shown in Figure 5.42b. This
operation can be performed by starting either at the middle or at the end of the
workpiece. A large reduction in the cross-sectional area can be achieved by reduc-
ing the feed of the workpiece. The bite (i.e., the length of feed before the working
stroke) ranges between 40 and 75 percent of the width of the forging die.

Piercing operation. A piercing operation is performed in order to obtain blind or
through holes in the billet. A through hole can be pierced directly in a short billet
in a single stroke by employing a punch and a supporting ring, as shown in Fig-
ure 5.42c. On the other hand, billets with large height-to-diameter ratios are
pierced while located directly on the die with the help of a piercer and possibly
an extension piece as well, as shown in Figure 5.42d. In this latter case, the di-
ameter of the piercer must not exceed 50 percent of that of the billet. For larger
holes, hollow punches are employed. Also, holes can be enlarged by tapered
punches.

5.5 Forging

179

4. Cutting off. Cutting off involves cutting the workpiece into separate parts using a
forge cutter or a suitable chisel. This is usually done in two stages, as can be seen
in Figure 5.42e.

5. Bending. In bending, thinning of the metal occurs on the convex side at the point
of localized bending (where bending actually takes place). It is, therefore, recom-
mended to upset the metal at this location before bending is performed, as shown
in Figure 5.42f, in order to obtain a quality bend.

Examples of open-die forged parts. As mentioned before, a part may require a series
of operations so that it can be given the desired shape by smith forging. Following are
some examples of smith-forged industrial components, together with the steps in-
volved in the manufacture of each part:

1. Large motor shaft. First, 24-inch-square (60 cm) steel ingots are rolled into
square blooms, each having a 12-inch (30-cm) side. The blooms are then heated
and hammered successively across the corners until the workpiece is finally
rounded to a diameter of 10 inches (25 cm). These steps are illustrated in Fig-
ure 5.43.

2. Flange coupling. The sequence of operations is illustrated in Figure 5.44. There are
two operations or stages involved, upsetting and heading. In heading, the flow of
metal of most of the billet is restricted by using a ring-shaped tool. This process al-
lows excellent grain flow to be obtained, which is particularly advantageous in car-
rying tangential loads.

FIGURE 5.43

The production of a
large motor shaft by
smith forging

Rotate -

Rotate

C? a

l^»^^ J L^ — >. 1 L/ — ■ 1

FIGURE 5.44

The production of
flange coupling by
smith forging

m m

J 1

180

Metal Forming

3. Rings. A billet is first upset and is then subjected to a piercing operation. This is
followed by an expanding operation using a mandrel to reduce the thickness of
the ring and increase its diameter as required. Larger rings are usually expanded
on a saddle. The steps involved in the process of ring forging are illustrated in
Figure 5.45.

Equipment for smith forging. Smaller billets are usually smith-forged using pneu-
matic-power hammers. Larger components are worked in steam-power hammers (or
large pneumatic hammers), whereas very large and heavy parts are produced by em-
ploying hydraulic presses. Following is a brief description of smith-forging equipment:

1. Steam-power hammers. A steam-power hammer consists mainly of the moving
parts (including the ram, the rod, and the piston); a lifting and propelling device,
which is a double-acting high-pressure steam cylinder; the housing or frame, which
can be either an arch or an open type; and the anvil. Figure 5.46 illustrates the
working principles. First, the piston and the other moving parts are raised by ad-
mitting steam into the lower side of the cylinder (under the piston) through the

FIGURE 5.45

The production of large
rings by smith forging

Saddle

Upsetting

Piercing

FIGURE 5.46

The working principles
of a steam-power
hammer

Steam

5.5 Forging 181

sliding valve. When a blow is required, the lever is actuated; the sliding valve is ac-
cordingly shifted to admit steam to the upper side of the cylinder (above the piston)
and exhaust the steam that was in the lower side, thus pushing the moving parts
downward at a high speed. In steam-power hammers, the velocity of impact can be
as high as 25 feet per second (3 m/s), whereas the mass of the moving parts can be
up to 11,000 slugs (5000 kg). The amount of energy delivered per blow is, there-
fore, extremely large and can be expressed by the equation:

E = '/2 mV2 (5.15)

where: E is the energy

m is the mass of the moving parts
V is the impact velocity

Nevertheless, not all of that energy is consumed in the deformation of the work-
piece. The moving parts rebound after impact, and the anvil will try to move in the
opposite direction, thus consuming or actually wasting a fraction of the blow en-
ergy. The ratio between the energy absorbed in deforming the metal to that deliv-
ered by the blow is called the efficiency of a hammer and can be given by the
following equation:

M 7

T\=—J—{\-K2) (5.16)

M + m

where: M is the mass of the anvil

A' is a factor that depends upon the elasticity of the billet

The harder and more elastic the billet is, the higher that factor will be, and the
lower the efficiency becomes. In addition, the hammer efficiency depends upon the
ratio MI{M + m), or actually the ratio between the masses of the anvil and the mov-
ing parts, which is taken in practice between 15 and 20. On the other hand, the
value of K ranges between 0.05 and 0.25.

2. Pneumatic-power hammers. There are two kinds of pneumatic-power hammers.
The first kind includes small hammers in which the air compressor is built in; they
usually have open frames because their capacity is limited. The second kind of
pneumatic hammer is generally similar to a steam-power hammer in construction
and operation, the only difference being that steam is replaced by compressed air (7
to 8 times the atmospheric pressure). As is the case with steam, this necessitates
separate installation for providing compressed air. Pneumatic hammers do not have
some of the disadvantages of steam hammers, such as dripping of water resulting
from condensation of leakage steam onto the hot billet. This may result in cracking
of the part, especially when forging steel.

3. Hydraulic presses. Heavy forgings are worked in hydraulic presses. The press in-
stallation is composed of the press itself and the hydraulic drive. Presses capable of
providing a force of 75,000 tons (750 MN) are quite common. Still, hydraulic
presses that are commonly used in the forging industry have capacities ranging be-
tween 1000 tons (10 MN) and 10,000 tons (100 MN). These presses can success-

182 5 Metal Forming

fully handle forgings weighing between 8 and 250 tons. The large-capacity presses
require extremely high oil pressure in the hydraulic cylinders (200 to 300 times the
atmospheric pressure). Because no pump can deliver an adequate oil discharge at
that pressure level, this process is usually overcome by employing accumulators
and intensifies that magnify the oil pressure delivered by the pump by a factor of
40 or even 60.

Planning the production of a smith-forged part. Before actually smith forging a part,
all the details of the process must be thoroughly planned. This involves preparation
of the design details, calculation of the dimensions and the weight of the stock and
of the product, choosing the forging operations as well as their sequence, choosing
tools and devices that will be used, and thinking about the details of the heating and
cooling cycles.

The first step in the design process is to draw the finished part and then obtain the
drawing of the forging by adding a machining as well as a forging allowance all
around. The machining allowance is the increase in any dimension to provide excess
metal that is removed by machining. This subsequent machining is required to remove
scales and the chilled, defected surface layers. The forging allowance is added mainly
to simplify the shape of the as-forged part. It is always recommended to make the
shape of a forging symmetrical and confined by plane and cylindrical surfaces. At this
stage, a suitable tolerance is assigned to each dimension to bring the design process to
an end.

The next step is to choose the appropriate equipment. Two factors affect the deci-
sion: the size of the forging and the rate of deformation (strain rate). Usually, forgings
weighing 2 tons or more are forged in hydraulic presses. Also, small forging made of
high-alloy steels and some nonferrous alloys must be forged on a press because they
are sensitive to high strain rates that arise when using power-hammer forging. At this
point, the manufacturing engineer is in a position to decide upon operations, tools, de-
vices, and the like needed to accomplish the desired task.

Closed-Die Forging

Closed-die forging involves shaping the hot forging stock in counterpart cavities or im-
pressions that have been machined into two mating halves of a die set. Under impact
(or squeezing), the hot metal plastically flows to fill the die cavity. Because the flow of
metal is restricted by the shape of the impressions, the forged part accurately conforms
to the shape of the cavity, provided that complete filling of the cavity is achieved.
Among the various advantages of closed-die forging are the greater consistency of
product attributes than in casting, the close tolerances and good surface finish with
minimum surplus material to be removed by machining, and the greater strength at
lower unit weight compared with castings or fabricated parts. In fact, the cost of parts
produced by machining (only) is usually two to three times the cost of closed-die forg-
ings. Nevertheless, the high cost of forging dies (compared with patterns, for example)
is the main shortcoming of this process, especially if intricate shapes are to be pro-
duced. Therefore, the process is recommended for mass or large-lot production of steel
and nonferrous components weighing up to about 900 pounds (350 kg).

5.5 Forging 183

Generally, there are two types of closed-die forging: conventional (or flash) die
forging and flashless die forging. In conventional flash die forging, the volume of the
slug has to be slightly larger than that of the die cavity. The surplus metal forms a flash
(fin) around the parting line. In flashless forging, no fin is formed, so the process con-
sequently calls for accurate control of the volume of the slug. If the slug is smaller than
the required final product, proper filling of the die cavity is not achieved. On the other
hand, when the size of the slug is bigger than that of the desired forging, excessive
load buildup will eventually result in the breaking of the tooling and/or equipment. Ac-
cordingly, flashless-forging dies are fitted with load-limiting devices to keep the gen-
erated load below a certain safe value in order to avoid breakage of the tooling.

In addition to shaping the metal in die cavities, the manufacturing cycle for a die-
forged part includes some other related operations, such as cutting or cropping the
rolled stock into slugs or billets, adequately heating the slugs, forging the slugs, trim-
ming the flash (in conventional forging), heat treating the forgings, descaling, and, fi-
nally, inspecting or quality controlling. The forging specifications differ from one
country to another; however, in order to ensure the product quality, one or more of the
following acceptance tests must be passed:

1. Chemical composition midway between the surface and the center

2. Mechanical properties

3. Corrosion tests

4. Nondestructive tests like magnetic detection of surface or subsurface hair cracks

5. Visual tests such as macroetch and macroexamination and sulfur painting for steel

Closed-die forging processes can be carried out using drop forging hammers, me-
chanical crank presses, and forging machines. Factors such as product shape and tol-
erances, quantities required, and forged alloys play an important role in determining
the best and most economical equipment to be employed in forging a desired product
as each of the processes has its own advantages and limitations. Following is a brief
description of the different techniques used in closed-die forging.

Drop forging. In drop forging, a type of closed-die forging, the force generated by the
hammer is caused by gravitational attraction resulting from the free fall of the ram. The
ram may be lifted by a single-acting steam (or air) cylinder or by friction rollers that en-
gage a board tightly fastened to the ram. In this latter type, called a board hammer, once
the ram reaches a predetermined desired height, a lever is actuated, the rollers retract,
and the board and ram fall freely to strike the workpiece. Figure 5.47 illustrates the
working principles. Whether a board hammer or single-acting steam hammer is used,
accurate matching of the two halves of the die (i.e., the impressions) must be ensured.
Therefore, the hammers employed in drop forging are usually of the double-housing (or
arch) type and are provided with adequate ram guidance. The desired alignment of the
two halves of the die is then achieved by wedging the upper half of the die onto the ram
and securing the lower half onto a bolster plate that is, in turn, tightly mounted on the
anvil. Also, the ratio of the weights of the anvil and the moving parts can go as high as
30 to 1 to ensure maximum efficiency and trouble-free impact.

184

5 Metal Forming

FIGURE 5.47

The working principles
of a board hammer

Wooden
board

Friction
roller

/-^Z^\

/-^^\

Drop-forging dies can have one, two, or several impressions, depending upon the
complexity of the required product. Simple shapes like gears, small flywheels, and
straight levers are usually forged in dies with one or two impressions, whereas prod-
ucts with intricate shapes are successively worked in multiple-impression dies, thus
making it possible to preshape a forging before it is forged into its final form. Opera-
tions like edging, drawing out, fullering, and bending are performed, each in its as-
signed impression. Finally, the desired shape is imparted to the metal in a finishing
impression that has exactly the same shape as the desired product; its dimensions are
slightly larger because shrinkage due to cooling down must be taken into account. As
can be seen in Figure 5.48, a gutter for flash is provided around the finishing impres-
sions. When properly designed, the gutter provides resistance to the flow of metal into

FIGURE 5.48

A gutter providing a
space for excess metal

Gutter

A forging in

the finishing

impression

Upper die half

Lower die half

Flash

5.5 Forging

185

it, thus preventing further flow from the impression and forcing the metal to fill all the
details, such as corners (which are the most difficult portions to fill).

The drop-forging process may involve several blows so that the desired final
shape of the forged part can be obtained. Lubricants are applied to ensure easy flow of
the metal within the cavity and to reduce friction and die wear. As many as four blows
may be needed while the part is in the finishing impressions, and the part should be
lifted slightly between successive blows to prevent overheating of the die. Finally, the
gas pressure forces the part out of the die. The number of blows delivered when the
part is in the different preshaping impressions is 1 Vi to 2 times the number of blows
while the part is in the finishing impression. This sequence of drop-forging operations
is shown when forging a connecting rod. As can be seen in Figure 5.49, the heated
stock is first placed in the fullering impression and then hammered once or twice to ob-
tain local spreading of the metal on the expanse of its cross section. The stock is then
transferred to the edging impression, where the metal is redistributed along its length
in order to properly fill the finishing die cavities (i.e., metal is "gathered" at certain
predetermined points and reduced at some other ones). This is usually achieved
through a series of blows, together with turnovers of the metal, as required. The next
operation in this sequence is bending, which may or may not be needed, depending
upon the design of the product. The stock is then worked in the semifinishing, or
blocking, impression before it is finally forged into the desired shape in the finishing
impression. We can see that the blocking operation contributes to reducing the tool
wear in the finishing impression by giving the part its general shape.

Press forging. Press forging, which is usually referred to as hot pressing, is carried
out using mechanical (crank-type) or hydraulic presses. These exert force at relatively
slow ram travel, resulting in steadily applied pressure instead of impacting pressure.

FIGURE 5.49

A multiple-impression
die and the forging
sequence for a
connecting rod

Initial forging stock

Blocking

Finishing

186

Metal Forming

FIGURE 5.50

Flash and flashless hot
pressing

Flash

Forging

Ejector

Flash hot pressing

Forging

Ejector
Flashless hot pressing

The nature of metal deformation during hot pressing is, therefore, substantially differ-
ent from that of drop forging. Under impact loading, the energy is transmitted into only
the surface layers of the workpiece, whereas, under squeezing (steadily applied pres-
sure), deformation penetrates deeper so that the entire volume of the workpiece simul-
taneously undergoes plastic deformation. Although multiple-impression dies are used,
it is always the goal of a good designer to minimize the number of impressions in a
die. It is also considered good industrial practice to use shaped blanks or preforms,
thus enabling the part to be forged in only a single stroke.

Hot pressing involves both flash as well as flashless forging. In both cases, the
forged part is pushed out of the die cavity by means of an ejector, as is illustrated in
Figure 5.50. Examples of some hot-pressed parts are shown in Figure 5.51, which also
shows the sequence of operations, the production rate, the estimated die life, and the
approximate production cost.

A characterizing feature of hot pressing is the accurate matching of the two halves
of a die due to the efficient guidance of the ram. Also, the number of working strokes
per minute can be as high as 40 or even 50. There is also the possibility of automating
the process through mechanization of blank feeding and of forging removal. It can,
therefore, clearly be seen that hot pressing has higher productivity than drop forging
and yields parts with greater accuracy in terms of tolerances within 0.010 to 0.020 inch
(0.2 up to 0.5 mm), less draft, and fewer design limitations. Nevertheless, the initial
capital cost is higher compared with drop forging because the cost of a crank press is
always higher than that of an equivalent hammer and because the process is economi-
cal only when the equipment is efficiently utilized. The difficulty of descaling the
blanks is another shortcoming of this process. However, this disadvantage can be elim-
inated by using hydraulic descaling (using a high-pressure water jet) or can be origi-
nally avoided by using heating furnaces with inert atmosphere.

Die forging in a horizontal forging machine. Although originally developed for head-
ing operations, the purpose of this machine has been broadened to produce a variety of
shapes. For instance, all axisymmetric parts such as rods with flanges (with through

5.5 Forging

187

FIGURE 5.51

Examples of hot-pressed parts

Break lever

Aluminum

Die life

Cost in cents per piece
4.9

(Bicycle)

alloy

40,000 pieces

Slug(H)

Forming

Trimming

Sizing ^^O*'

Piercing

Bearing race

(Bearing)

SAE-5 00

Die life
30,000

Cost in cents per piece
11.2

Slug

Upsetting

Piercing

Backward extrusion

,+0.5

h« 1 — *-i

73.2^

Sizing

1 ~l 28.88+° 5 mm

Valve

(Gas equipment)

Slug(H)

Brass

Upsetting

Die life
40,000

Forming

Cost in cents per piece
7.7

60.1

Trimming ™^1"

and blind holes) and/or side projections are commonly produced on horizontal forging
machines. A rolled stock is cut to length, heated in a heating unit, and automatically
fed to the machine. As can be seen in Figure 5.52, the hot part is then held by station-
ary grips (actually a split die) and upset by an upsetting ram or header. The process in-
volves mainly upsetting and gathering where the blank is first upset; then metal flows
to fill the die cavity, as opposed to drop forging, where it is spread or flattened. In the
return stroke, the upsetting ram retracts, and the part is removed or transferred to the
next impression of the horizontal forging machine. It is obvious that a part can be
forged in one or several cavities, depending upon the complexity of its shape.

The main advantage of this process is the high production rate (up to 5000 parts
per hour) due to the fact that it can be fully automated. Further advantages include the

188

Metal Forming

FIGURE 5.52

Die forging in a
horizontal forging
machine

Grip die

Upsetting
.tool

Ejector

Grip die

elimination of the flash and the forging draft and the high efficiency of material uti-
lization because the process involves little or no waste.

Recent Developments in Forging

Warm forging, high-energy-rate forging, and forming of metals in their mushy state are
among the important developments in forging technology. These newly developed
processes are usually carried out to obtain intricate shapes or unique structures that
cannot be obtained by conventional forging processes. Following is a brief description
of each of these processes, together with their advantages and disadvantages.

Warm forging. Warm forging involves forging of the metal at a temperature some-
what below the recrystallization temperature. This process combines some advantages
of both the hot and the cold forming processes while eliminating their shortcomings.
On one hand, increased plasticity and lower load requirements are caused by the rela-
tively high forging temperature. On the other hand, improved mechanical properties,
less scaling, and longer die life are due to the lower temperatures used as compared
with those used with hot forging.

High-energy-rate forging. The conventional forging process takes some time, during
which the hot metal cools down and its resistance to deformation increases. As this
does not occur with high-energy-rate forging (HERF), where the whole process is per-
formed within a few thousandths of a second, the hot metal does not have enough time
to cool down and heat is not dissipated into the surroundings. Therefore, HERF is very
successful when forging intricate shapes with thin sections. A special HERF machine
must be used. In fact, the Petro-Forge machine was developed at the Mechanical En-
gineering Department of Birmingham University in England for this reason, and a
bulky machine with the name Dynapak was developed in the United States. In the first
case, the machine consists mainly of an internal-combustion (IC) cylinder integrated
into the structure of a high-speed press. The IC cylinder is provided with a sudden re-
lease valve that allows the platen attached to the piston to be fired instantaneously
when the combustion pressure reaches a preset level. The four stages of the working

5.5 Forging

189

FIGURE 5.53

The working cycle of the
Petro-Forge

cycle of the Petro-Forge are shown in Figure 5.53. In the case of the Dynapak, high-
pressure nitrogen in a power cylinder is used to push the platen downward. Installa-
tions to produce and keep high-pressure gas are, therefore, required in this case.

Forging of alloys in their mushy state. Forging alloys in their mushy state involves
plastically forming alloys in the temperature range above the solidus line. Because
an alloy at that temperature consists partly of a liquid phase, a remarkable decrease
in the required forging load is experienced. The process also has some other merits,
such as the high processing rate and the high quality of products compared with
castings. Moreover, the friction at the billet-container interface has been found to be
almost negligible. Nevertheless, the process is still considered to be in its experi-
mental stage because of the instability of alloys having low solid fractions. Recently,
it was reported that progress has been made toward solving this problem at the In-
stitute of Industrial Science, Tokyo University, where the instability was overcome

Injection

Charging Working stroke

Return stroke

Oil sump

<*> J

il mist

Injection

At the beginning of the firing cycle the ram/piston assembly
(A) is held at the top of its stroke by low pressure air in the
back pressure chamber (B) closing the combustion chamber
porting by the seal (C), this being a cylindrical projection on
the top face of the piston (A). The exhaust valve (D) is open
and pressure in the combustion chamber (E) is atmospheric.
Upon pressing the firing button the fuel injection phase
starts; the exhaust valve (D) is closed and the gaseous fuel
is admitted into the combustion chamber (E) via the gas
valve (F).

Working stroke

As soon as the force due to the combustion pressure acting
on the small area (I) on top of the seal (C) is sufficiently
large to overcome the opposed force due to the low back
pressure in the space (B) acting on the annular lower face
of the piston, the piston (A) starts to move. As a result the
porting between the combustion chamber (E) and the cylinder
is opened and the gases are permitted to expand to act over
the whole piston area. This results in a large force surge acting
on the piston/ram assembly which is accelerated downwards
to impinge on the workpiece.

Charging

After closing the gas valve (F) the combustion chamber is
charged by admitting compressed air through the inlet valve
(G). As soon as charging is completed, the inlet valve (G) is
closed and the air/gas mixture is ignited by the spark plug
(H). This results in a seven to eightfold rise of the pressure
in the combustion chamber (E).

Return stroke

During the working stroke the back pressure in space (B) is
intensified and consequently acts as a return spring as soon as
the forming operation is completed, thus rapidly separating
the dies. The return of the ram/piston assembly to its initial
position is completed by the opening of the exhaust valve (D)
which permits gases to leave through the duct (J). The cycle
of operation is normally completed in one second.

190 5 Metal Forming

by dispersing a very fine alumina powder. This also yielded improved mechanical
properties of forgings.

Forgeability

For the proper planning of a forging process, it is important to know the deformation be-
havior of the metal to be forged with regard to the resistance to deformation and any an-
ticipated adverse effects, such as cracking. For this reason, the term forgeability was
introduced and can be defined as the tolerance of a metal for deformation without failure.
Although there is no commonly accepted standard test, quantitative assessment of the
forgeability of a metal (or an alloy) can be obtained through one of the following tests.

Upsetting test. The upsetting test involves upsetting a series of cylindrical billets
having the same dimensions to different degrees of deformation (reductions in height).
The maximum limit of upsettability without failure or cracking (usually peripheral
cracks) is taken as a measure of forgeability.

Notched-bar upsetting test. The notched-bar upsetting test is basically similar to the
first test, except that longitudinal notches or serrations are made prior to upsetting. It
is believed that this test provides a more reliable index of forgeability.

Hot-impact tensile test. A conventional impact-testing machine fitted with a tension-
test attachment is employed. A hot bar of the metal to be studied is tested, and the im-
pact tensile strength is taken as a measure of forgeability. This test is recommended
when studying the forgeability of alloys that are sensitive to high strain rates.

Hot twist test. The hot twist test involves twisting a round, hot bar and counting the
number of twists until failure. The greater the number of twists, the better the forge-
ability is considered to be. Using the same bar material, this test can be performed at
different temperatures in order to obtain the forging temperature range in which the
forgeability of a metal is maximum.

Forgeability of Some Alloys

It is obvious that the results of any of the preceding tests are affected by factors like
the composition of an alloy, the presence of impurities, the grain size, and the number
of phases present. These are added to the effect of temperature, which generally im-
proves forgeability up to a certain limit, where other phases start to appear or where
grain growth becomes excessive. At this point, any further increase in temperature is
accompanied by a decrease in forgeability. Following is a list indicating the relative
forgeability of some alloys in descending order (i.e., alloys with better forgeability are
mentioned first):

1. Aluminum alloys

2. Magnesium alloys

3. Copper alloys

4. Plain-carbon steels

5.5 Forging 191

5. Low-alloy steels

6. Martensitic stainless steel

7. Austenitic stainless steel

8. Nickel alloys

9. Titanium alloys

10. Iron-base superalloys

11. Cobalt-base superalloys

12. Molybdenum alloys

13. Nickel-base superalloys

14. Tungsten alloys

15. Beryllium

Lubrication in Forging

In hot forging, the role of lubricants is not just limited to eliminating friction and en-
suring easy flow of metal. A lubricant actually prevents the hot metal from sticking to
the die and meanwhile prevents the surface layers of the hot metal from being chilled
by the relatively cold die. Therefore, water spray, sawdust, or liners of relatively soft
metals are sometimes employed to prevent adhesion. Mineral oil alone or mixed with
graphite is also used, especially for aluminum and magnesium alloys. Graphite and/or
molybdenum disulfide are widely used for plain-carbon steels, low-alloy steels, and
copper alloys, whereas melting glass is used for difficult-to-forge alloys like alloy
steel, nickel alloys, and titanium.

Defects in Forged Products

Various surface and body defects may be observed in forgings. The kind of defect de-
pends upon many factors, such as the forging process, the forged metal, the tool de-
sign, and the temperature at which the process is carried out. Cracking, folds, and
improper sections are generally the defects observed in forged products. Following is
a brief description of each defect and its causes.

Cracking. Cracking is due to the initiation of tensile stresses during the forging
process. Examples are hot tears, which are peripheral longitudinal cracks experienced
in upsetting processes at high degrees of deformation, and center cavities, which occur
in the primary forging of low-ductility steels. Thermal cracks may also initiate in cases
when nonuniform temperature distribution prevails.

Folds. In upsetting and heading processes, folding is a common defect that is obvi-
ously caused by buckling. Folds may also be observed at the edges of parts produced
by smith forging if the reduction per pass is too small.

Improper sections. Improper sections include dead-metal zones, piping, and turbu-
lent (i.e., irregular or violent) metal flow. They are basically related to and caused by
poor tool design.

192

5 Metal Forming

Forging Die Materials

During their service life, forging dies are subjected to severe conditions such as high
temperatures, excessive pressures, and abrasion. A die material must, therefore, pos-
sess adequate hardness at high temperatures as well as high toughness to be able to
withstand the severe conditions. Special tool steels (hot-work steels including one or
more of the following alloying additives: chromium, nickel, molybdenum, and vana-
dium) are employed as die materials. Die blocks are annealed, machined to make the
shanks, hardened, and tempered; then, impression cavities are sunk by toolmakers.

Fundamentals of Closed-Die Forging Design

The range of forged products with respect to size, shape, and properties is very wide
indeed. For this reason, it is both advisable and advantageous for the product designer
to consider forging in the early stages of planning the processes for manufacturing new
products. The forging design is influenced not only by its function and the properties
of the material being processed but also by the kind, capabilities, and shortcomings of
the production equipment available in the manufacturing facilities. Therefore, it is im-
possible to discuss in detail all considerations arising from the infinite combinations of
the various factors. Nevertheless, some general guidelines apply in all cases and
should be strictly adhered to if a sound forging is to be obtained. Following are some
recommended forging design principles.

Parting line. The plane of separation between the upper and lower halves of a closed
die set is called the parting line. The parting line can be straight, whether horizontal or
inclined, or can be irregular, including more than one plane. The parting line must be
designated on all forging drawings as it affects the initial cost and wear of the forging
die, the grain flow that, in turn, affects the mechanical properties of the forging, and,
finally, the trimming procedure and/or subsequent machining operations on the fin-
ished part. Following are some considerations for determining the shape and position
of the parting line:

1. The parting line should usually pass through the maximum periphery of the forging
mainly because it is always easier to spread the metal laterally than to force it to fill
deep, narrow die impressions (see Figure 5.54).

FIGURE 5.54

Recommended location
of the parting line
(Courtesy of the
Aluminum Association,
Inc., Washington, D.C.)

Preferred

Less desirable

5.5 Forging

193

FIGURE 5.55

Flat-sided forging for
simplifying the die
construction (Courtesy
of the Aluminum
Association, Inc.,
Washington, D.C.)

Plane surface formed
/ by flat upper die

Parting line

Contour of forging

formed by impression

in bottom die

FIGURE 5.56

Using the parting line
to promote the
alignment of the fibrous
macrostructure
(Courtesy of the
Aluminum Association,
Inc., Washington, D.C.)

Grain structure is

ruptured at the

parting line

Parting line

Undesirable

These parting lines result in metal flow patterns
that cause forging defects

Most economical as all of
the impression is in one die

This parting line should not be —
above the center of the bottom web

Parting at the ends

of ribs results in

good grain structure

Recommended - The flow lines are smooth at stressed sections
with these parting lines

194

Metal Forming

2. It is always advantageous, whenever possible, to try to simplify the die construction
if the design is to end up with flat-sided forgings (see Figure 5.55). This will
markedly reduce the die cost because machining is limited to the lower die half.
Also, the possibility of mismatch between die halves is eliminated.

3. If an inclined parting line must exist, it is generally recommended to limit the in-
clination so that it does not exceed 75°. The reason is that inclined flashes may cre-
ate problems in trimming and subsequent machining.

4. A parting line should be located so that it promotes alignment of the fibrous
macrostructure to fulfill the strength requirement of a forging. Because excess
metal flows out of the die cavity into the gutter as the process proceeds, mislocat-
ing the parting line will probably result in irregularities, as can be seen in Figure
5.56, which indicates the fibrous macrostructures resulting from different locations
of the parting line.

5. When the forging comprises a web enclosed by ribs, as illustrated in Figure 5.57,
the parting line should preferably pass through the centerline of the web. It is also
desirable, with respect to the alignment of fibers, to have the parting line either at
the top or at the bottom surfaces. However, that desirable location usually creates
manufacturing problems and is not used unless the direction of the fibrous
macrostructure is critical.

6. If an irregular parting line must exist, avoid side thrust of the die, which will cause
the die halves to shift away from each other sideways, resulting in matching errors.
Figure 5.58 illustrates the problem of side thrust accompanying irregular parting
lines, together with two suggested solutions.

Draft. Draft refers to the taper given to internal and external sides of a closed-die forg-
ing and is expressed as an angle from the direction of the forging stroke. Draft is required
on the vast majority of forgings to avoid production difficulties, to aid in achieving desired
metal flow, and to allow easy removal of the forging from the die cavity. It is obvious that

FIGURE 5.57

Location of the parting
line with respect to a
web (Courtesy of the
Aluminum Association,
Inc.. Washington, D.C.)

Parting line

A—1

Section AA

Section BB

5.5 Forging

195

FIGURE 5.58

The problem of side
thrust accompanying
irregular parting lines
and two suggested
solutions (Courtesy of
the Aluminum
Association, Inc.,
Washington, D.C.)

Die lock

Impractical — Side thrust makes it difficult to hold the dies
in match accurately

Die lock

Not recommended — Dies with counterlocks are expensive to
build and troublesome to maintain

Die lock

— Upper die

Forging
plane

- Forging

Bottom die

Upper die

Forging
plane

Counterlock

Forging
Bottom die

Upper die

Forging
plane

Forging
Bottom die

Preferred — The best method is to incline the forging with
respect to the forging plane

the smaller the draft angle, the more difficult it is to remove the forging out of the die. For
this reason, draft angles of less than 5° are not permitted if the part is to be produced by
drop forging (remember that there is no ejector to push the part out). Standard draft angles
are 7°, 5°, 3°, 1 °, and 0°. A draft angle of 3° is usually used for metal having good forge-
ability, such as aluminum and magnesium, whereas 5° and 7° angles are used for steels,
titanium, and the like. It is a recommended practice to use a constant draft all over the pe-
riphery of the forging. It is also common to apply a smaller draft angle on the outside pe-
riphery than on the inside one. This is justified in that the outer surface will shrink away
from the surface of the die cavity as a result of the part's cooling down, thus facilitating
the removal of the forging. Following are some useful examples and guidelines:

1. When designing the product, try to make use of the natural draft inherent in some
shapes, such as curved and conical surfaces (see Figure 5.59).

2. In some cases, changing the orientation of the die cavity may result in natural draft,
thus eliminating the need for any draft on the surfaces (see Figure 5.60).

196

5 Metal Forming

FIGURE 5.59

Examples of the natural

draft inherent in some
shapes

Parting /
line v

Parting
line

FIGURE 5.60

Examples of changing
the orientation of the
impression to provide
natural draft

Parting
line

FIGURE 5.61

Methods for matching
the contours of two die
impressions having
different depths:
(a) increasing the
dimension of the upper
surface; (b) using a
pad; (c) employing a
matching draft
(Courtesy of the
Aluminum Association,
Inc., Washington, D.C.)

Sometimes, the cavity in one of the die halves (for instance, the upper) is shallower
than that in the other half. This may create problems in matching the contours of
the two die halves at the parting line. It is, therefore, recommended that one of the
three methods illustrated in Figure 5.61a, b, or c be used. The first method involves
keeping the draft the same as in the lower cavity but increasing the dimension of
the upper surface of the cavity. This results in an increase in weight, and this solu-

5° (Ref)

Parting line

:p[

«- Dim (applies to
lower side only)

(a)

Parting line

!*- Dim (applies to
both side)
(b)

5°"~

Match
draft

«t ;

Parting line \

5°—

Dim (ar.

)plies tc

note that it applies to the
original 5° intersection and
not to the subsequent match
draft intersection)

(c)

5.5 Forging

197

tion is limited to smaller cavities. The second method is based on keeping the draft
constant in both halves by introducing a "pad" whose height varies between 0.06
inch (1.5 mm) and 0.5 inch (12.5 mm), depending upon the size of the forging. The
third method, which is more common, is to provide greater draft on the shallower
die cavity; this is usually referred to as matching draft.

Ribs. A rib is a thin part of the forging that is normal to (or slightly inclined to) the
forging plane. It is obvious that optimized lighter weight of a forging calls for reduc-
ing the thickness of long ribs. However, note that the narrower and longer the rib is,
the higher the forging pressure is and the more difficult it is to obtain a sound rib. It is
actually a common practice to keep the height-to-thickness ratio of a rib below 6,
preferably at 4. The choice of a value for this ratio depends upon many factors, such
as the kind of metal being processed and the forging geometry (i.e., the location of the
rib, the location of the parting line, and the fillet radii). Figure 5.62 indicates the de-
sirable rib design as well as limitations imposed on possible alternatives.

Webs. A web is a thin part of the forging that is passing through or parallel to the
forging plane (see Figure 5.63). Although it is always desirable to keep the thickness
of a web at the minimum, there are practical limits for this. The minimum thickness
of webs depends on the kind of material being worked (actually on its forging tem-
perature range), the size of forging (expressed as the net area of metal at the parting
line), and on the average width. Table 5.3 indicates recommended web thickness val-
ues applicable to precision and conventional aluminum forgings. For blocking cavi-
ties, the values given in Table 5.3 must be increased by 50 percent. Also, for steels
and other metals having poorer forgeability than aluminum, it is advisable to increase
the values for web thickness. Thin webs may cause unfilled sections, may warp in
heat treatment, and may require additional straightening operations; they even cool
faster than the rest of the forging after the forging process, resulting in shrinkage,
possible tears, and distortion.

FIGURE 5.62

Parting

Recommended rib

Defect

line

design (Courtesy of the

location

v$\

Aluminum Association,

Parting

\Il_

Inc., Washington, D.C.)

W//////A

line

Avoid
and

thin-ledged ribs
small fillet radii

Recommended

Possible defect
if a> b

198

Metal Forming

FIGURE 5.63

The shape of a web in
forging (Courtesy of the
Aluminum Association,
Inc., Washington. D.C.)

Corner radii. There are two main factors that must be taken into consideration when
selecting a small value for a corner radius. First, a small corner radius requires a sharp
fillet in the die steel, which acts as a stress raiser; second, the smaller the corner radius,
the higher the forging pressure required to fill the die cavity. In addition, some other
factors affect the choice of the corner radius, such as the distance from the corner to
the parting line and the forgeability of the metal being worked. The larger the distance
from the parting line, the larger the corner radius should be. Also, whereas a corner ra-
dius of 0.0625 inch (1.5 mm) is generally considered adequate for aluminum forging,
a corner radius of at least 0.125 inch (3 mm) is used for titanium forgings of similar
shape and size. In addition, the product designer should try to keep the corner radii as
consistent as possible and avoid blending different values for a given shape in order to
reduce the die cost (because there will be no need for many tool changes during die
sinking). Corner radii at the end of high, thin ribs are critical. A rule of thumb states

5.5 Forging

199

TABLE 5.3

Recommended size of
minimum web
thickness

Up to Average Width
in. (m)

Up to Cross-Sectional Area
in.2 (m2)

Web Thickness
in. (mm)

3 (0.075)

10 (0.00625)

0.09 (2.25)

4 (0.1)

30 (0.01875)

0.12 (3)

6 (0.15)

60 (0.0375)

0.16 (4)

8 (0.2)

100 (0.0625)

0.19 (4.75)

11 (0.275)

200 (0.125)

0.25 (6.25)

14 (0.35)

350 (0.21875)

0.31 (7.75)

18 (0.45)

550 (0.34375)

0.37 (9.25)

22 (0.55)

850 (0.53125)

0.44 (11)

26 (0.65)

1200 (0.75)

0.50 (12.5)

34 (0.85)

2000 (1.25)

0.62 (15.5)

41 (1.025)

3000 (1.875)

0.75 (18.75)

47 (1.1175)

4000 (2.50)

1.25 (31.25)

52 (1.3)

5000 (3.125)

2.00 (50)

that it is always desirable to have the rib thickness equal to twice the value of the cor-
ner radius. A thicker rib may have a flat edge with two corner radii, each equal to the
recommended value. Figure 5.64 illustrates these recommendations regarding corner
radii for ribs.

Fillet radii. It is of supreme importance that the product designer allow generous radii
for the fillets because abrupt diversion of the direction of metal flow can result in nu-
merous defects in the product. Figure 5.65 indicates the step-by-step initiation of forg-
ing defects and shows that small fillets result in separation of the metal from the die
and initiation of voids. Although these can be filled at a later stage, laps and cold shuts
will replace these voids. When the shape of the part to be forged is intricate (i.e., in-
volving thin ribs and long, thin webs), the metal may preferentially flow into the gut-
ter rather than into the die cavity. This results in a shear in the fibrous macrostructure
and is referred to as flow-through. This latter defect can be avoided by using larger-
than-normal fillets.

FIGURE 5.64

Recommendations
regarding corner radii
for ribs (Courtesy of the
Aluminum Association,
Inc., Washington, D.C.)

Straight

-ur*

Recommended
values of radii

t = 2/?
Recommended

Acceptable

200

5 Metal Forming

FIGURE 5.65

Defects caused by
employing smaller fillet
radii (Courtesy of the
Aluminum Association
Inc., Washington, D.C.)

Large fillets Forging stock

Die motion

Metal does not

hug
i sharp corner

Metal reaches
bottom of

cavity before
ing section

These cold shuts
flawed in the
forging

Punchout holes. Punchout holes are through holes in a thin web that are produced
during, but not after, the forging process. Punchouts reduce the net projected area of
the forging, thus reducing the forging load required. If properly located and designed,
they can be of great assistance in producing forgings with thin webs. In addition to the
manufacturing advantages of punchouts, they serve functional design purposes, such
as reducing the mass of a forging and/or providing clearance. Following are some
guidelines regarding the design of punchouts:

1. Try to locate a punchout around the central area of a thin web, where the frictional
force that impedes the metal flow is maximum.

2. Whenever possible, use a gutter around the interior periphery of a punchout. This
provides a successful means for the surplus metal to escape.

3. A single large punchout is generally more advantageous than many smaller ones
that have the same area. Accordingly, try to reduce the number of punchouts unless
more are dictated by functional requirements.

5.6 Cold Forming Processes 201

4. Although punchouts generally aid in eliminating the problems associated with the
heat treatment of forgings, it may prove beneficial to take the limitations imposed
by heat treatment processes into account when designing the contour of a punchout
(i.e., try to avoid irregular contours with sharp corners).

Pockets and recesses. Pockets and recesses are used to save material, promote the
desirable alignment of the fibrous macrostructure, and improve the mechanical proper-
ties by reducing the thickness, thus achieving a higher degree of deformation. Follow-
ing are some guidelines:

1. Recesses should never be perpendicular to the direction of metal flow.

2. Recesses are formed by punches or plugs in the dies. Therefore, the recess depth is
restricted to the value of its diameter (or to the value of minimum transverse di-
mension for noncircular recesses).

3. Simple contours for the recesses, together with generous fillets, should be tried.

OLD FORMING PROCESSES

Cold forming processes are employed mainly to obtain improved mechanical proper-
ties, better surface finish, and closer tolerances. Several cold forming techniques have
found wide industrial application. Among these are sizing, swaging, coining, and cold
heading. Following is a brief description of each of them.

Sizing

Sizing (see Figure 5.66a) is a process in which the metal is squeezed in the forming di-
rection but flows unrestricted in all transverse directions. This process is used primar-
ily for straightening forged parts, improving the surface quality, and obtaining accurate
dimensions. A sizing operation can ensure accuracy of dimensions within 0.004 up to
0.010 inch (0.1 up to 0.25 mm). Meanwhile, the pressure generated on the tools can go
up to 180,000 pounds per square inch (1300 MN/m2).

Swaging

Swaging (see Figure 5.66b) involves imparting the required shape and accurate di-
mensions to the entire forging (or most of it). Usually, swaging is carried out in a die
where a flash is formed and subsequently removed by abrasive wheels or a trimming
operation. Note that the flow of metal in the swaging process is more restricted than in
sizing. Accordingly, higher forming pressures are experienced and can go up to
250,000 pounds per square inch (1800 MN/m2).

Coining

Coining (see Figure 5.66c) is a process in which the part subjected to coining is com-
pletely confined within the die cavity (by the die and the punch). The volume of the
original forging must be very close to that of the finished part. Any tangible increase
in that volume may result in excessive pressures and the breakage of tools. Still, com-

202

Metal Forming

FIGURE 5.66

Cold forming
processes: (a) sizing;
(b) swaging; (c) coining

(b)

mon pressures (even when no problems are encountered) are in the order of 320,000
pounds per square inch (2200 MN/nV). For this reason, coining processes (also sizing
and swaging) are carried out on special presses called knuckle presses. The main
mechanism of a knuckle press is shown in Figure 5.67. It is characterized by the abil-
ity to deliver a large force with a small stroke of the ram.

Cold Heading

Cold heading is used to manufacture bolts, rivets, nuts, nails, and similar parts with
heads and collars. A group of typical products are illustrated in Figure 5.68. The main
production equipment involves a multistage automatic cold header that operates on the

5.6 Cold Forming Processes

203

FIGURE 5.67

The working principles
of a knuckle press for
cold forming processes

.\\\\\K\\\\N

same principle as a horizontal forging machine. Full automation and high productivity
are among the advantages of this process. Products having accurate dimensions can be
produced at a rate of 30 to 300 pieces per minute. Starting from coiled wires or rods
made of plain-carbon steel and nonferrous metals with diameters ranging from 0.025
to 1.6 inches (0.6 to 40 mm), blanks are processed at different stations. Feeding, trans-
fer, and ejection of the products are also automated. Figure 5.69 illustrates the differ-
ent stages involved in a simple cold heading operation.

Lubrication in Cold Forming

Lubricants employed in cold forming are similar to those used in heavy wire-drawing
processes. Phosphating followed by soap dipping is successful with steels, whereas
only soap is considered adequate for nonferrous metals.

FIGURE 5.68

Some products
manufactured using an
automatic cold header

204

Metal Forming

FIGURE 5.69

Different stages of a
simple cold heading
operation

3- €

Review Questions

1. Why have metal forming processes gained
widespread industrial application since World
War II?

2. What are the two main groups of metal form-
ing processes?

3. List the different factors affecting the defor-
mation process. Tell how each influences de-
formation.

4. Why are cold forming processes always ac-
companied by work-hardening, whereas hot
forming processes are not?

5. What is meant by the fibrous macro structure?

6. Are the mechanical properties of a rolled sheet
isotropic? Why?

7. What is meant by the state of stress? List the
three general types.

8. List some advantages of hot forming. What are
some disadvantages?

9. List some advantages of cold forming. What
are some disadvantages?

10. What may happen when a large section of
steel is heated at a rapid rate? Why?

11. What should be avoided when heating large
steel sections prior to hot forming?

12. Where does friction occur in metal forming?

13. What are the harmful effects of friction on the
forming process?

14. Is friction always harmful in all metal forming
processes?

15. Can lead be used as a lubricant when forming
copper? Why?

16. When forming lead at room temperature, do
you consider it cold forming? Why?

17. Why are lubricants used in metal forming
processes? List some useful effects.

18. List some lubricants used in cold forming
processes.

19. List some lubricants used in hot forming
processes.

20. Which do you recommend for further process-
ing by machining, a cold-worked part or a hot-
worked part?

21. Is hot rolling the most widely used metal
forming process? Why?

22. List some of the useful effects of hot rolling.

23. Define rolling.

24. What is the angle of contact?

25. For heavier sections, would you recommend
larger angles of contact in rolling? Why?

26. What is the state of stress in rolling?

27. List the different types of rolling mills.

28. What are the different parts of a roll? What is
the function of each?

Explain why Sendzimir mills are used.

What are universal mills used for?

31. List three groups included in the range of
rolled products.

29
30

Chapter 5 Review Questions

205

32. Explain, using sketches, how seamless tubes
are manufactured.

33. What is alligatoring? What causes it?

34. Define wire drawing.

35. Which mechanical property should the metal
possess if it is to be used in a drawing
process? Why?

36. What is the state of stress in drawing?

37. List some advantages of the drawing process.

38. How is a metal prepared for a drawing process?

39. What are the different zones in a drawing die?

40. Mention the range of the apex angles (of con-
ical shapes) used in drawing dies.

41. What material do you recommend to be used
in making drawing dies?

42. Describe a draw bench.

43. What kinds of lubricants are used in drawing
processes?

44. What is the drawing ratio?

45. Give an expression indicating the reduction
achieved in a wire-drawing process.

46. Why do internal bursts occur in wire-drawing
processes?

47. What are arrowhead fractures and why do they
occur?

48. What is the state of stress in tube drawing?

49. Using sketches, illustrate the different tech-
niques used in tube drawing.

50. Define extrusion.

51. Why can extrusion be used with metals having
relatively poor plasticity?

52. List some advantages of the extrusion process.

53. What are the shortcomings and limitations of
the extrusion process?

54. Using sketches, differentiate between the di-
rect and indirect extrusion techniques.

55. Although indirect extrusion almost eliminates
friction, it is not commonly used in industry.
Why?

56. List the advantages of hydrostatic extrusion.

57. Compare extrusion with rolling with respect to
efficiency of material utilization.

58. When is conventional direct extrusion recom-
mended as a production process?

59. Describe impact extrusion.

60. Why is the leading end of an extruded section
always sheared off?

61. What are dead-metal zones?

62. If hardness measurements are taken across the
section (say, circular) of an extruded part,
what locations will have higher hardness val-
ues? Can you plot hardness versus distance
from the center?

63. What lubricants can be used in cold extrusion?

64. What material do you recommend as a lubri-
cant when hot extruding stainless steel?

65. What defect may occur when extruding mag-
nesium at low extrusion ratios?

66. What is piping and why does it occur?

67. In extrusion dies, what is meant by the circle
size?

68. List some considerations that must be taken
into account when designing a section for ex-
trusion.

69. Why should a designer try to avoid sharp cor-
ners at the root of a die tongue? Explain using
neat sketches.

70. As a product designer, you are given a very in-
tricate section for production by extrusion. Is
there any way around this problem without
being forced to use a die with a very intricate
construction? How?

71. List some considerations for the design of im-
pact extrusions.

72. How can you avoid shear failure at the bottom
of the wall of an impact extrusion?

73. Does forging involve just imparting a certain
shape to a billet?

206

Metal Forming

75.

76.
77.

78.
79.

74. Is it just a matter of economy to produce a
crankshaft by forging rather than by machin-
ing from a solid stock? Why?
Can a metal such as aluminum be forged at
any temperature? Why?
List the main types of forging processes.
Which process is suited for the production of
small batches of large parts?

Give examples of parts produced by each type

of forging process. Support your answer with

evidence.

What is the modern version of blacksmi thing?

What are the different operations involved in

that process?

80. When do you recommend using a power-actu-
ated hammer as a forging machine? Mention
the type of forging process.

81. For which type of forging is a drop hammer
employed?

82. For which type of forging is a crank press
employed?

Using sketches, illustrate the different stages
in manufacturing a ring by forging.

List the advantages that forging has over cast-
ing when producing large numbers of small
parts having relatively complex shapes.

85. In the comparison of Question 84, what are
the shortcomings of forging? Why don't they
affect your decision in that particular case?

86. List some of the specified acceptance tests to
be performed on forgings.

87. What is a board hammer used for?

88. Is it true that a closed type of forging die can
have only one impression? Explain why.

83.

89. What does hot pressing mean?

90. What is the advantage of HERF?

91. What are the advantages of warm forging?

92. What is meant by a mushy state?

93. Define forgeability. How can it be quantita-
tively assessed?

94. What is the most forgeable metal?

95. What is the main role of lubricants in hot forg-
ing?

96. As a product designer, how can you manipu-
late the alignment of the fibrous macrostruc-
ture?

97. List some guidelines regarding the location of
the parting line between the upper and lower
halves of a die set.

98. What is meant by the term draft in forging?

99. A die was designed to forge an aluminum part.
Can the same design be used to forge a similar
part made of titanium? Why?

100. Explain the meaning of matching draft, using
sketches.

101. Differentiate between a web and a rib in a
forging.

102. What is the difference between a corner radius
and a fillet radius? Use sketches.

103. What are punchout holes in a forging?

104. List some advantages of including punchout
holes in a forging design.

105. Why are recesses sometimes included in a
forging design?

106. List the different cold forming processes and
use sketches to illustrate how they differ.

Chapter 5 Design Example

207

PiohLems

1. In hot rolling, determine the load on each roll of
a two-high rolling mill, given the following:

Diameter of the

roll:

Stock width:

Initial thickness:

Final thickness:

Flow stress of

rolled material:

20 inches (500 mm)
48 inches (1020 mm)
0.08 inch (2 mm)
0.04 inch (1 mm)

14,200 lb/in.2 (100 MN/m2)

In hot rolling low-carbon-steel plate 48 inches
( 1 200 mm) in width, given the roll diameter as 20
inches (500 mm), initial thickness as 1.5 inches
(37.5 mm), final thickness as 0.4 inch (10 mm),
and the flow stress of steel as 28,400 lb/in.2 (200
MN/m2), calculate the number of rolling passes if
the maximum load on the roll in each pass is not
to exceed 225,000 pounds force (1.0 MN).

3. Write a computer program to solve Problem 2,
assuming that all the data are variables to be
given for each design.

4. Calculate the maximum achievable reduction in
a single drawing of a lead wire.

5. Estimate the largest possible extrusion ratio of
2.0-inch (50-mm) aluminum bar having mean
flow stress of 21,900 lb/in.2 (150 MN/m2) if the
press available has a capacity of only 45,000
pounds force (200 kN).

6. Plot a curve indicating the efficiency of a drop
hammer versus the ratio between the weights of
the anvil and the moving parts if the value of K
that represents the elasticity of the billet is taken
as 0. 1 . What ratio do you suggest? Why should it
not be justified to take large ratios?

Design Example

PROBLEM

Design a simple wrench that measures 1/2 inch (12.5 mm) across bolt-head flats and
is used for loosening nuts and bolts. The torque required to loosen (or tighten) a bolt
(or a nut) is 1 lb ft (6.8 Nm). The production volume is 25,000 pieces per year. Forg-
ing is recommended as a manufacturing process.

Solution

Because the wrench is going to be short, it cannot be held by the full hand but prob-
ably by only three fingers. The force that can be exerted is to be taken, therefore, as
4 pounds. The arm of the lever is equal to (1 x 12)/4, or 3 inches (75mm). Add on
allowance for the holding fingers. The shape of the wrench will be as shown in Fig-
ure 5.70.

Now, let us select the materials. A suitable material would be AISI 1045 CD steel
to facilitate machining (sawing) of the stock material. Closed-die forging of the billets

208

5 Metal Forming

FIGURE 5.70

A wrench manufactured
by forging

Section AA

1875 inch

0.6 inch

R= 0.95 inch

0.375 inch

;3e

■Parting line

0.25 inch

is recommended, as well as employing drop-forging hammers. To facilitate withdrawal
of the part, the cross section of the handle should be elliptical (see Figure 5.70). The
parting line should coincide with the major axis of the ellipse.
Let us check the stress due to bending:

/ = -n a3b = - (7t)(0.375)3(0.1875) = 7.7 x 10"
4 4

where: a is half the major axis
b is half the minor axis

3 in.4

stress =

Ma
1

5 x 12 x 0.375

= 2922 lb/in.

/ / 7.7 x 10"3

It is less than the allowable stress for 1045 CD steel, which is

^°°° = 30,000 1M„.'

In order to check the bearing stress, let us assume a shift of 0.25 inch between the
forces acting on the faces of the nut to form a couple (this assumption can be verified
if we draw the nut and the wrench to scale):

each force =

60
0.25

= 240 pounds

Further assume that the bearing area is 0.375 by 0.25 inch. The bearing stress is,
therefore,

240

= 2560 lb/in/

0.375 x 0.25
It is less than the allowable stress of the 1045 CD steel.

Chapter 5 Design Projects 209

The forged wrench finally has to be trimmed and then machined on the surfaces
indicated in Figure 5.70. An allowance of 1/64 inch should be provided between the
wrench open-head and the nut. Now, our design is complete and ready to be released
to the workshop.

ssign Projects

BcL

1. A clock frame 3 by 5 inches (75 by 125 mm) is manufactured by machining an
aluminum-alloy stock. Make a design and a preliminary feasibility study so that it
can be produced by extrusion. Assume the production volume is 20,000 pieces per
year.

2. A motor frame that has a 6-inch (150-mm) internal diameter and that is 10 inches
(250 mm) long is currently produced by casting. That process yields a high per-
centage of rejects, and the production cost is relatively high. Knowing that the pro-
duction volume is 20,000 pieces per year, redesign the part so that it will be lighter
and can be easily produced by an appropriate metal forming operation that has a
high efficiency of material utilization.

3. A pulley transmits a torque of 600 lb ft (816 Nm) to a shaft that is IV4 inches (31
mm) in diameter. It is to be driven by a flat belt that is 2 inches (50 mm) in width.
Provide a detailed design for the pulley if the production volume is 10,000 pieces
per year and the pulley is manufactured by forging.

4. A connecting lever is to be manufactured by forging. The estimated production vol-
ume is 50,000 pieces per year. The lever has two short bosses, each at one of its
ends, and each has a vertical hole 3/4 inch (19 mm) in diameter. The horizontal dis-
tance between the centers of the two holes is 12 inches (300 mm), and the vertical
difference in levels is 3 inches (75 mm). The lever during its functioning is sub-
jected to a bending moment of 200 lb ft (272 Nm). Make a detailed design for this
lever.

5. If the lever in Problem 4 is to be used in a space vehicle, would you use the same
material? What are the necessary design changes? Make a design appropriate for
this new situation.

6. Design a gear blank that transmits a torque of 200 lb ft (272 Nm) to a shaft that is
3/4 inch (19 mm) in diameter. The pitch diameter of the gear is 8 inches (200 mm),
and 40 teeth are to be cut in that blank by machining. Assume the production vol-
ume is 10,000 pieces per year.

7. A straight-toothed spur-gear wheel transmits a torque of 1200 lb ft (1632 Nm) to
a steel shaft (AISI 1045 CD steel) that is 2 inches (50 mm) in diameter. The pitch

210 5 Metal Forming

diameter of the gear is 16 inches (400 mm), its width is 4 inches (100 mm), and
the base diameter is 15 inches (375 mm). Make a complete design for this gear's
blank (i.e., before teeth are cut) when it is to be manufactured by forging. Assume
the production volume is 10,000 pieces per year.

A shaft has a minimum diameter of 1 inch (25 mm) at both its ends, where it is
to be mounted in two ball bearings. The total length of the shaft is 12 inches
(300 mm). The shaft is to have a gear at its middle, with 40 teeth and a pitch-
circle diameter of 1.9 inches (47.5 mm). The width of the gear is 2 inches (50 mm).
Make a design for this assembly if the production volume is 50,000 per year.

Chapter 6

eet Metal
orking

INTRODUCTION

The processes of sheet metal working have recently gained widespread indus-
trial application. Their main advantages are their high productivity and the close
tolerances and excellent surface finish of the products (which usually require
no further machining). The range of products manufactured by these processes
is vast, but, in general, all of these products have thin walls (relative to their
surface area) and relatively intricate shapes. Sheets made from a variety of
metals (e.g., low-carbon steel, high-ductility alloy steel, copper and some of its
alloys, and aluminum and some of its alloys) can be successfully worked into
useful products. Therefore, these processes are continually becoming more at-
tractive to the automotive, aerospace, electrical, and consumer goods indus-
tries. Products that had in the past always been manufactured by processes
like casting and forging have been redesigned so that they can be produced by
sheet metal working. Components like pulleys, connecting rods for sewing ma-
chines, and even large gears are now within the range of sheet metal products.

Sheet metals are usually worked while in their cold state. However, when
processing thick sheets, which are at least 0.25 inch (6 mm) and are referred
to as plates, thermal cutting is employed to obtain the required blank shape,
and the blank is then hot-worked in a hydraulic or friction screw press. Thus,
fabrication of boilers, tanks, ship hulls, and the like would certainly require hot
working of thick plates.

By far, the most commonly used operations in sheet metal working are
those performed in a press. For this reason, they are usually referred to as

211

212

6 Sheet Metal Working

press working, or simply stamping, operations. Other techniques involve high-
energy-rate forming (HERF), like using explosives or impulsive discharges of
electrical energy to form the blank, and spinning of the sheet metal on a form
mandrel. This chapter will describe each of the various operations employed in
sheet metal working.

6.1 PRESS WORKING OPERATIONS

All press working operations of sheet metals can be divided into two main groups: cut-
ting operations and shape-forming operations. Cutting operations involve separating a
part of the blank, whereas forming operations involve nondestructive plastic deforma-
tion, which causes relative motion of parts of the blank with respect to each other. Cut-
ting operations include shearing, cutoff, parting, blanking, punching, and notching.
Shape-forming operations include various bending operations, deep drawing, emboss-
ing, and stretch-forming.

Cutting Operations

The mechanics of separating the metal are the same in all sheet metal cutting operations.
Therefore, the operations are identified according to the shape of the curve along which
cutting takes place. When the sheet metal is cut along a straight line, the operation is
called shearing and is usually performed using inclined blades or guillotine shears in
order to reduce the force required (see Figure 6.1). Cutting takes place gradually, not all
at once, over the width of the sheet metal because the upper blade is inclined. The angle
of inclination of the upper blade usually falls between 4° and 8° and must not exceed 15°
so that the sheet metal is not pushed out by the horizontal component of the reaction.

When cutting takes place along an open curve (or on an open corrugated line), the
operation is referred to as cutoff, provided that the blanks match each other or can be
fully nested, as shown in Figure 6.2. The cutoff operation results in almost no waste of
stock and is, therefore, considered to be very efficient with respect to material utiliza-
tion. This operation is usually performed in a die that is mounted on a crank press. If the
blanks do not match each other, it is necessary for cutting to take place along two open
curves (or lines), as shown in Figure 6.3. In this case, the operation is called parting. It

FIGURE 6.1

Shearing operation with
inclined blades

Upper

>ZL blade

6.1 Press Working Operations

213

FIGURE 6.2

Examples of cutoff
operations

Strip

lyi*

Cutting takes

place along these

two lines, each

stroke

Blank

Final
blank shape

FIGURE 6.3

An example of a parting
operation

Cutting takes place
simultaneously
along these lines

Blank

is clear from the figure that a parting operation results in some waste of stock and is,
therefore, less efficient than shearing and cutoff operations.

In blanking operations, cutting occurs along a closed contour and results in a
relatively high percentage of waste in stock metal, a fact that makes blanking oper-
ations less efficient than other cutting operations. Nevertheless, this process is used
for mass production of blanks that cannot be manufactured by any of the preceding
operations. An efficient layout of blanks on the strip of sheet metal can result in an
appreciable saving of material. An example of a good layout is shown in Figure
6.4a, where circular blanks are staggered. The in-line arrangement shown in Figure
6.4b is less efficient in terms of material utilization. Because a blanking operation
is performed in a die, there is a limit to the minimum distance between two adja-
cent blanks. It is always advantageous to keep this minimum distance larger than 70
percent of the thickness of the sheet metal. In blanking, the part separated from the
sheet metal is the product, and it is usually further processed. But if the remaining

FIGURE 6.4

Two methods for laying
out circular blanks for
blanking operations:

(a) staggered layout;

(b) in-line arrangement

Blank

°3%

Strip

Narrower
strip

ooo
ooo

QOO

Blank

(a)

(b)

214

6 Sheet Metal Working

FIGURE 6.5

Different patterns of
holes produced by
perforating operations

o
o o
o o
o o

o o o o o

FIGURE 6.6

Progressive working
operations

Seminotching
(2) \ (3)

Punching
pilot holes

imw m

Notching

Final
product

Cut off along this line
to separate the product

part of the sheet is required as a product, the operation is then termed punching.
Sometimes, it is required to simultaneously punch a pattern of small holes as an or-
nament, for light distribution, or for ventilation; the operation is then referred to as
perforating. Figure 6.5 illustrates some patterns of perforated holes.

A notching operation is actually a special case of punching, where the removed
part is adjacent to the edge of the strip. It is clear that any required shape can be ob-
tained by carrying out several notching operations. For this reason, notching is usually
employed in progressive dies. A similar operation, called seminotching, in which the
separated part is not attached to the side of the strip, is also used in progressive work-
ing of sheet metals. In Figure 6.6, we can see both of these operations and how they
can be employed progressively to produce a blank with an intricate shape.

Mechanics of sheet metal cutting. Let us now look further at the process of cutting
sheet metal. For simplicity, consider the simple case where a circular punch, together
with a matching die, are employed to punch a hole. Figure 6.7 shows the punch, die,
and sheet metal during a punching operation. When a load is applied through the
punch, the upper surface of the metal is elastically bent over the edge of the punch,
while the lower surface is bent over the edge of the die. With further increase in the
punch load, the elastic curvature becomes permanent or plastic and is referred to as the
rollover. Next, the punch sinks into the upper surface of the sheet, while the lower sur-
face sinks into the die hole. This stage involves mainly plastic flow of metal by shear-
ing as there are two forces equal in magnitude and opposite in direction, subjecting the
cylindrical surface within the metal to intense shear stress. The result will be a cylin-
drical smooth surface in contact with the cylindrical surface of the punch as it sinks
into the sheet metal. Also, a similar surface forms the border of the part of the metal
sinking into the die hole. Each of these smooth surfaces is called a burnish. The extent
of a burnish depends upon the metal of the sheet as well as on the design features of

6.1 Press Working Operations

215

FIGURE 6.7

Stages of a blanking
operation

Punch

Crack

Final hole

Final blank

Burr

Fracture surface

Burnish

Rollover

FIGURE 6.8

Blanking operations

where the punch-die

clearance is:

(a) excessive; (b) too

tight

the die. The burnish ranges approximately between 40 and 60 percent of the stock
thickness, the higher values being for soft ductile materials like lead and aluminum. At
this stage, two cracks initiate simultaneously in the sheet metal, one at the edge of the
punch and the other at the edge of the die. These two cracks propagate and finally meet
each other to allow separation of the blank from the sheet metal. This zone has a rough
surface and is called the fracture surface (break area). Finally, when the newly formed
blank is about to be completely separated from the stock, a burr is formed all around
its upper edge. Thus, the profile of the edge of a blank involves four zones: a rollover,
a burnish, a fracture surface, and a burr. In fact, the profile of the edge of the gener-
ated hole consists of the same four zones, but in reverse order.

We are now in a position to discuss the effects of some process parameters, such
as the punch-die clearance. Figure 6.8a illustrates the case where the punch-die clear-
ance is excessive and is almost equal to the thickness of the sheet. Initially, the metal
is bent onto the round edges of the punch and the die, and it then forms a short circu-
lar wall connecting the flat bottom and the bulk of the sheet. With further increase in
the applied load, the wall elongates under the tensile stress, and tearing eventually oc-
curs. As can be seen in Figure 6.8a, the blank resulting in this case has a bent, torn
edge all around and, therefore, has no value. On the other hand, if the punch-die clear-
ance is too tight, as shown in Figure 6.8b, the two cracks that initiate toward the end
of the operation do not meet, and another shearing must take place so that the blank
can be separated. This operation is referred to as the secondary shear. As can be seen,
the obtained blank has an extremely rough side. In addition, the elastically recovering

Location where

secondary shear

occurs

ESf-FS3

Edge of
blank

(b)

(a)

216

6 Sheet Metal Working

FIGURE 6.9

Elastic recovery of the
metal around the hole
gripping the punch

FIGURE 6.10

Elastic recovery of the
blank necessitating die
relief

sheet stock tends to grip the punch, as shown in Figure 6.9, thus increasing the force
required to withdraw the punch from the hole, which is usually called the stripping
force. This results in excessive punch wear and shorter tool life. On the other hand, the
blank undergoes elastic recovery, and it is, therefore, necessary to provide relief by en-
larging the lower part of the die hole, as shown in Figure 6.10.

Between these two extremes for the punch-die clearance, there exists an optimum
value that reduces or minimizes the stripping force and the tool wear and also gives a
blank with a larger burnish and smaller fracture surface. This recommended value for
the punch-die clearance is usually taken as about 10 to 15 percent of the thickness of
the sheet metal, depending upon the kind of metal being punched.

Forces required. Based on the preceding discussion, the force required for cutting
sheet metal is equal to the area subjected to shear stress (the product of the perimeter
of the blank multiplied by the thickness of the sheet metal) multiplied by the ultimate
shear strength of the metal being cut. The blanking force can be expressed by the fol-
lowing equation:

F=KxQxtx xultimate (6.1)

where: Q is the perimeter
/ is the thickness
^ultimate is trie ultimate shear strength

Note that K is an experimentally determined factor to account for the deviation of the
stress state from that of pure shear and is taken as about 1.3. The ultimate shear stress
can either be obtained from handbooks or be taken as approximately 0.8 of the ulti-
mate tensile strength of the same metal.

We can now see that one of the tasks of a manufacturing engineer is to calculate the
required force for blanking (or punching) and to make sure that it is below the capacity
of the available press. This is particularly important in industries that involve blanking
relatively thick plates. There is, however, a solution to the problem when the required
force is higher than the capacity of the available press. It is usually achieved by bevel-
ing (or shearing) the punch face in punching operations and the upper surface of the die
steel in blanking operations. Shearing the punch results in a perfect hole but a distorted
blank, whereas shearing the die yields a perfect blank but a distorted hole. Nevertheless,
in both cases, cutting takes place gradually, not all at once, along the contour of the hole
(or the blank), with the final outcome being a reduction in the required blanking force.
The shear angle is usually taken proportional to the thickness of the sheet metal and
ranges between 2° and 8°. Double-sheared punches are quite common and are employed

6.1 Press Working Operations

217

to avoid the possibility of horizontal displacement of sheet metals during punching. Fig-
ure 6. 1 1 illustrates the basic concept of punch and die shearing. It also provides a sketch
of a double-sheared punch.

Another important aspect of the punching (or blanking) operation is the stripping
force (i.e., the force required to pull the punch out of the hole). It is usually taken as
10 percent of the cutting force, although it depends upon some process parameters,
such as the elasticity and plasticity of the sheet metals, the punch-die clearance, and
the kind of lubricant used.

Bar cropping. Bar cropping is similar to sheet metal cutting. Although bars, not
sheets, are cut, the mechanics of the process are similar to those of sheet metal cutting,
and separation of the cropped part is due to plastic flow caused by intense shear stress.
The process is used for mass production of billets for hot forging and cold forming
processes. Nevertheless, the distortion and work-hardening at the sheared cross section
limit the application of bar cropping when the billets are to be cold formed. Therefore,
a modified version of the cropping operation has to be used. It involves completely
confining the cropped billet and applying an axial stress of approximately 20 percent
of the tensile strength of the bar material. This bar-cropping technique, which is shown
in Figure 6.12, yields a very smooth cropped surface and distortion-free billets.

Fine blanking. As we saw previously, the profile of the edge of a blank is not smooth
but consists of four zones: the rollover, the burnish, the fracture surface (break area),
and the burr. Sometimes, however, the blank must have a straight, smooth side for
some functional reasons. In this case, an operation called fine blanking is employed, as

FIGURE 6.11

Shearing of the punch

and the die:

(a) sheared punch

resulting in distorted

blanks; (b) sheared die

resulting in distorted

holes

FIGURE 6.12

Bar cropping with
workpiece totally
confined

Movable

Fixed
blade

218

6 Sheet Metal Working

FIGURE 6.13

Fine-blanking operation

Upper punch

Pressure pad

Sheet metal

Die steel

Figure 6.13 shows. This operation necessitates the use of a triple-action press and a
special die with a very small punch-die clearance. As can be seen in the figure, the
metal is squeezed and restrained from moving in the lateral directions in order to con-
trol the shear flow along a straight vertical direction. A variety of shapes can be pro-
duced by this method. They can have any irregular outer contour and a number of
holes as well. The fine-blanking operation has found widespread application in preci-
sion industries.

Miscellaneous cutting operations. The primary operation that is used for preparing
strips for blanking is needed because the available sheets vary in width between 32 and
80 inches (800 to 2000 mm), a range that is usually not suitable because of the di-
mensions of the die and the press. Therefore, coils having a suitable width have to be
obtained first. The operation performed is called slitting, and it employs two circular
cutters for each straight cut. Sometimes, slitting is carried out in a rolling plant, and
coils are then shipped ready for blanking.

A secondary operation that is sometimes carried out on blanks (or holes) to elim-
inate rough sides and/or to adjust dimensions is the shaving operation. The excess
metal in this case is removed in the form of chips. As can be seen in Figure 6.14, the
punch-die clearance is very small. For this reason, the die must be rigid, and matching
of its two halves must be carefully checked.

Sometimes, punching operations are mistakenly called piercing. In fact, the me-
chanics of sheet metal cutting in the two operations are completely different. We can
see in Figure 6.15 that piercing involves a tearing action. We can also see the pointed

FIGURE 6.14

The shaving operation

Sheet
meta

6.1 Press Working Operations

219

FIGURE 6.15

The piercing operation

Punch

ie steel

shape of the punch. Neither blanks nor metal waste result from the piercing operation.
Instead, a short sleeve is generated around the hole, which sometimes has functional
application in toy construction and the like.

Cutting-die construction. The construction of cutting dies may take various forms.
The simplest one is the drop-through die, which is shown in Figure 6.16. In addition
to the punch and die steels, the die includes the upper and lower shoes, the guideposts,
and some other auxiliary components for guiding and holding the metal strip. The
stripper plate touches the strip first and holds it firmly during the blanking operation;
it then continues to press it until the punch is totally withdrawn from the hole made in
the strip. The generated blanks fall through the die hole, which has a relief for this rea-
son, and are collected in a container located below the bed of the press.

Consequently, this die construction is applicable only if the bed of the press has a
hole. On the other hand, if the diameter of the required blanks is too large, the use of
a drop-through die may result in a defect called dishing. As shown in Figure 6.17. this
defect involves slackening of the middle of the blank in such a manner that it becomes
curved and not flat. The answer to this problem lies in employing a return-type die.

FIGURE 6.16

Die construction for
simple drop-through
blanking die

Spring

Stripper
plate

Die steel

Lower
die shoe

220

6 Sheet Metal Working

FIGURE 6.17

A vertical section
through a blank with
the dishing defect

Figure 6.18 shows that in this type of die construction, the blank is supported through-
out the operation by a spring-actuated block that finally pushes the blank upward
above the surface of the strip, where it is automatically collected. A more complicated
die construction, like that shown in Figure 6.19, can be used to perform two operations
simultaneously. This is usually referred to as a compound die. As can be seen in Fig-
ure 6.19, the hollow blanking punch is also a hole-punching die. This allows blanking
and punching to be carried out simultaneously. The product, which is a washer, and the
central scrap are removed by return blocks.

Bending Operations

Bending is the simplest operation of sheet metal working. It can, therefore, be carried
out by employing simple hand tools. As opposed to cutting operations, there is always
a clear displacement between the forces acting during a bending operation. The gener-
ated bending moment forces a part of the sheet to be bent with respect to the rest of it
through local plastic deformation. Therefore, all straight unbent surfaces are not sub-
jected to bending stresses and do not undergo any deformation. Figure 6.20 illustrates
the most commonly used types of bending dies: the V-type, the wiping, and the chan-
nel (U-type) dies. We can see that the displacement between forces is maximum in the

FIGURE 6.18

A return-type die

Punch

Stripper
plate

FIGURE 6.19

A compound die for
producing a washer

Punch steel

Stripper
plate

Required
washer

Die steel

6.1 Press Working Operations

221

FIGURE 6.20

The three common
types of bending dies:
(a) V-type die; (b) wiping
die; (c) channel (U-type)
die

mzzzzzszEEzmtz

(a)

(b)

case of the V-type die, and, therefore, lower forces are required to bend sheet metal
when using this kind of die.

Mechanics of bending. The bending of sheet metal resembles the case of a beam with
a very high width-to-height ratio. When the load is applied, the bend zone undergoes
elastic deformation; then plastic deformation occurs with a further increase in the ap-
plied load. During the elastic deformation phase, the external fibers in the bend zone
are subjected to tension, whereas the internal fibers are subjected to compression. The
distribution of stresses is shown in Figure 6.21a. Note that there is a neutral plane that
is free of stresses at the middle of the thickness of the sheet. The length of the neutral
axis remains constant and does not undergo either elongation or contraction. Next,
when the plastic phase starts, the neutral plane approaches the inner surface of the
bend, as can be seen in Figure 6.21b. The location of the neutral plane is dependent
upon many factors, such as the thickness of the sheet metal, the radius, and the degree
of bend. Nevertheless, the distance between the neutral plane and the inner surface of
the bend is taken as equal to 40 percent of the thickness of the sheet metal as a first ap-
proximation for blank-development calculations.

Let us now consider a very important phenomenon — namely, springback, which
is an elastic recovery of the sheet metal after the removal of the bending load. As Fig-
ure 6.22 indicates, for bending by an angle of 90°, the springback amounts to a few de-
grees. Consequently, the obtained angle of bend is larger than the required one. Even

FIGURE 6.21

Distribution of stress
across the sheet
thickness: (a) in the
early stage of bending;
(b) toward the end of a
bending operation

Bending
moment

Neutral
plane

Y-

Tension

Compression

Sheet
metal

(a)

(b)

222

6 Sheet Metal Working

FIGURE 6.22

The springback
phenomenon

Position of the sheet metal
after partial elastic recovery

Springback

toward the end of the bending operation, the zone around the neutral plane is subjected
to elastic stresses and, therefore, undergoes elastic deformation (see Figure 6.21b). As
a result, the elastic core tries to return to its initial flat position as soon as the load is
removed. When doing so, it is impeded by the plastically deformed zones. The final
outcome is, therefore, an elastic recovery of just a few degrees. Consequently, the way
to eliminate springback involves forcing this elastic core to undergo plastic deforma-
tion. This can be achieved through either of the techniques shown in Figure 6.23a and
b. In the first case, the punch is made so that a projection squeezes the metal locally;
in the second case, high tensile stress is superimposed upon bending. A third solution
is overbending, as shown in Figure 6.23c. In this case, the amount of overbending
should be equal to the springback so that the exact required angle is obtained after the
elastic recovery.

Blank development. We have previously referred to the fact that the neutral plane
does not undergo any deformation during the bending operation and that its length,
therefore, remains unchanged. Accordingly, the length of the blank before bending can
be obtained by determining the length of the neutral plane within the final product. The
lengths of the straight sections remain unchanged and are added together. The follow-
ing equation can be applied to any general bending product, such as the one shown in
Figure 6.24:

L = total length of blank before bending

3 4 180 ' 180 2 180 3

(6.2)

FIGURE 6.23

Methods used to
eliminate springback:

(a) bottoming;

(b) overbending;

Bending moment

The final
required
position

6.1 Press Working Operations

223

FIGURE 6.24

A bending product
divided into straight
and circular sections
for blank development

where: R is equal to r + 0.4/

r is the inner radius of a bend

t is the thickness of the sheet metal

R is the radius of the neutral axis

Classification of bending operations. Various operations can be classified as bending,
although each one has its own industrial name. They include, for example, conven-
tional bending, flanging, hemming, wiring, and corrugating. The flanging operation is
quite similar to conventional bending, except that the ratio of the lengths of the bent
part to that of the sheet metal is small. Flanging is usually employed to avoid a sharp
edge, thus eliminating the possibility of injury. It is also used to add stiffness to the
edges of sheet metal and for assembly purposes.

Among the bending operations, hemming used to be a very important one, before
the recent developments in welding and can-forming technologies. A hem is a flange
that is bent by 180°; it is used now to get rid of a sharp edge and to add stiffness to
sheet metal. A few decades ago, hems were widely employed for seaming sheet met-
als. Figure 6.25 shows four different kinds of hems. A similar operation is wiring,
which is shown in Figure 6.26. True wiring involves bending the edge of the sheet
metal around a wire. Sometimes, the operation is performed without a wire, and it is
then referred to as false wiring.

Corrugating is another operation that involves bending sheet metal. Different
shapes, like those shown in Figure 6.27, are obtained by this operation. These shapes
possess better rigidity and can resist bending moments normal to the corrugated cross

FIGURE 6.25

Different kinds of hems

Flat hem

Open hem

Teardrop hem

Seaming using
two hems

224

6 Sheet Metal Working

FIGURE 6.26

Wiring operation

True wiring

False wiring

FIGURE 6.27

Different shapes of
corrugated sheet metal

JU%*

section mainly because of the increase in the moment of inertia of the section due to
corrugation and because of the work-hardened zones resulting from bending.

Miscellaneous bending operations. Conventional bending operations are usually car-
ried out on a press brake. However, with the developments in metal forming theories
and machine tool design and construction, new techniques have evolved that are em-
ployed in bending not only sheet metal but also iron angles, structural beams, and
tubes. Figure 6.28 illustrates the working principles and the stages involved in roll
bending. As can be seen in the figure, the rolls form a pyramid-type arrangement. Two
rolls are used to feed the material, whereas the third (roll B) gradually bends it (see
Figure 6.28a and b). The direction of feed is then reversed, and roll A now gradually
bends the beam (see Figure 6.28c and d).

Another bending operation that recently emerged and that is gaining industrial ap-
plication is rotary bending. Figure 6.29 illustrates the working principles of this oper-

FIGURE 6.28

Stages involved in roll
bending a structural
beam: (a) feeding;

(b) initial bending;

(d) reversing the
direction of feed

Roll A

Roll B

(b)

(d)

FIGURE 6.29

Working principles of
rotary bending

Saddle

Rocker

Anvil

6.1 Press Working Operations

225

ation. As can be seen, the rotary bender includes three main components: the saddle,
the rocker, and the die anvil. The rocker is actually a cylinder with a V-notch along its
length. The rocker is completely secured inside the saddle (i.e., the saddle acts like a
housing) and can rotate but cannot fall out. The rotary bender can be mounted on a
press brake. The rocker acts as both a pressure pad and a bending punch. Among the
advantages claimed for rotary bending are the elimination of the pressure pad and its
springs (or nitrogen cylinders), lower required tonnage, and the possibility of over-
bending without the need for any horizontal cams. This new method has been patented
by the Accurate Manufacturing Association and is nicknamed by industrial personnel
as the "Pac Man" bending operation.

A bending process that is usually mistakenly mentioned among the rolling
processes is the manufacture of thin-walled welded pipes. Although rolls are the form-
ing tools, the operation is actually a gradual and continuous bending of a strip that is
not accompanied by any variation in the thickness of that strip. Figure 6.30 indicates
the basic principles of this process. Notice that the width of the strip is gradually bent
to take the form of a circle. Strip edges must be descaled and mechanically processed
before the process is performed to improve weldability. Either butt or high-frequency
induction welding is employed to weld the edges together after the required circular
cross section is obtained. This process is more economical and more productive than
seamless tube rolling. Poor strength and corrosion resistance of seams are considered
as its main disadvantages.

Deep Drawing Operation

Deep drawing involves the manufacture of deep, cuplike products from thin sheet
metal. As can be seen in Figure 6.31, the tooling basically involves a punch with a
round corner and a die with a large edge radius. It can also be seen that the punch-die
clearance is slightly larger than the thickness of the sheet metal. When load is applied
through the punch, the metal is forced to flow radially and sink into the die hole to
form a cup. This is an oversimplification of a rather complex problem. For the proper
design of deep-drawn products as well as the tooling required, we have to gain a
deeper insight into the process and understand its mechanics.

Mechanics of deep drawing. Consider what happens during the early stages of ap-
plying the load. As Figure 6.32a shows, the blank is first bent onto the round edge of
the die hole. With further increase in the applied load, the part of the blank that was
bent is straightened in order to sink into the annular clearance between the punch and

FIGURE 6.30

Roll bending as
employed in the
manufacture of seamed
tubes

r"-M

-i-

m
i

r~\

KlS

ntn

rfi
r

11441

rf!t

1WJ

226

6 Sheet Metal Working

FIGURE 6.31

Basic concept of deep
drawing

Blank holder
The drawn cup

Die

the die, thus forming a short, straight, vertical wall. Next, the rest of the blank starts to
flow radially and to sink into the die hole, but because the lower surface of the blank
is in contact with the upper flat surface of the die steel, frictional forces try to impede
that flow. These forces are a result of static friction; their magnitude drops as the blank
metal starts to move. Now consider what happens to a sector of the blank, such as that
shown in Figure 6.32b, when its metal flows radially. It is clear that the width of the
sector shrinks so that the large peripheral perimeter of the blank can fit into the smaller
perimeter of the die hole. This is caused by circumferential compressive stresses act-
ing within the plane of the blank. With further increase in the applied load, most of the
blank sinks into the die hole, forming a long vertical wall, while the remaining part of
the blank takes the form of a small annular flange (see Figure 6.31). The vertical wall
is subjected to uniaxial tension whose magnitude is increasing when going toward the
bottom of the cup.

We can see from the preceding discussion that the deep drawing process involves
five stages: bending, straightening, friction, compression, and tension. Different parts
of the blank being drawn are subjected to different states of stress. As a result, the de-
formation is not even throughout the blank, as is clear in Figure 6.33, which shows an
exaggerated longitudinal section of a drawn cup. While the flange gets thicker because
of the circumferential compressive stress, the vertical wall gets thinner, and thinning is

FIGURE 6.32

Mechanics of deep

drawing: (a) first stage

of deep drawing (i.e.,

bending);

(b) compression stage

in deep drawing

(a)

(b)

6.1 Press Working Operations

227

FIGURE 6.33

An exaggerated
longitudinal section of a
drawn cup, with the
states of stress at
different locations

Maximum thinning
occurs here

Biaxial
G compression,
thickening

Uniaxial tension,
thinning

maximum at the lowest part of the wall adjacent to the bottom of the cup. Accordingly,
if the cup is broken during the drawing process, failure is expected to occur at the lo-
cation of maximum thinning. An upper bound for the maximum drawing force can,
therefore, be given by the following equation:

F = K x (d + t)tCT (6.3)

where: F is the maximum required drawing force

d is the diameter of the punch

t is the thickness of the blank

<3T is the ultimate tensile strength of the blank material

The blank holder. As previously mentioned, the thin blank is subjected to compres-
sive stresses within its plane. This is similar to the case of a slender column subjected
to compression, where buckling is expected to occur if the slenderness ratio (i.e.,
length/thickness) is higher than a certain value. Therefore, by virtue of similarity, if the
ratio of the diameter of the blank to its thickness exceeds a certain value, buckling oc-
curs. Actually, if (D0 -d)lt> 18, where D is the blank diameter, d is the punch diam-
eter, and t is the thickness, the annular flange will buckle and crimple. This is a product
defect referred to as wrinkling.

One way to eliminate wrinkling (buckling) of the thin blank is to support it over
its entire area. This is done by sandwiching the blank between the upper surface of the
die steel and the lower surface of an annular ring that exerts pressure upon the blank,
as shown in Figure 6.31. This supporting ring is called the blank holder, and the force
exerted on it can be generated by die springs or a compressed gas like nitrogen. On the
other hand, higher frictional forces will initiate at both the upper and lower surfaces of
the blank as a result of the blank-holding force. For this reason, lubricants like soap in
water, waxes, mineral oil, and graphite are applied to both surfaces of the blank. More-
over, the upper surface of the die steel as well as the lower surface of the blank holder
must be very smooth (ground and lapped). As a rule of thumb, the blank-holding force
is taken as 1/3 the force required for drawing.

228 6 Sheet Metal Working

Variables affecting deep drawing. Now that we understand the mechanics of the
process, we can identify and predict the effect of each of the process variables. For ex-
ample, we can see that poor lubrication results in higher friction forces, and, accord-
ingly, a higher drawing force is required. In fact, in most cases of poor lubrication, the
cup cross section does not withstand the high tensile force, and failure of the wall at
the bottom takes place during the process. A small die corner radius would increase the
bending and straightening forces, thus increasing the drawing force, and the final out-
come would be a result similar to that caused by poor lubrication.

In addition to these process variables, the geometry of the blank has a marked ef-
fect not only on the process but also on the attributes of the final product. An appro-
priate quantitative way of expressing the geometry is the number indicating the
thickness as a percentage of the diameter, or (t/D) x 100. For smaller values of this
percentage (e.g., 0.5), excessive wrinkling should be expected, unless a high blank-
holding force is used. If the percentage is higher than 3, no wrinkling occurs, and a
blank holder is not necessary.

Another important variable is the drawing ratio, wJaich is given by the following
equation:

R = 4 <6-4>

where: R is the drawing ratio

D is diameter of the blank
d is the diameter of the punch

It has been experimentally found that the deep drawing operation does not yield a
sound cup when the drawing ratio is higher than 2 (i.e., for successful drawing, R must
be less than 2).

Another number that is commonly used to characterize drawing operations is the
percentage reduction. It can be given by the following equation:

r = ^^xl00 (6.5)

where: r is the percentage reduction
D is the diameter of the blank
d is the diameter of the punch

It is a common industrial practice to take the value of r as less than 50 percent in order
to have a sound product without any tearing. When the final product is long and neces-
sitates a value of r higher than 50 percent, an intermediate cup must be obtained first, as
shown in Figure 6.34. The intermediate cup must have dimensions that keep the per-
centage reduction below 50. It can then be redrawn, as illustrated in Figure 6.35, once or
several times until the final required dimensions are achieved. The maximum permissi-
ble percentage reduction in the redrawing operations is always far less than 50 percent.
It is usually taken as 30 percent, 20 percent, and 13 percent, in the first, second, and third
redraws, respectively. If several redrawing operations are required, the product should

6.1 Press Working Operations

229

FIGURE 6.34

The use of an
intermediate cup when
the total required
reduction ratio is high

t I ' 'I

r = D ~d X 100 > 50

? 9

/&ZZZZZZZZZZZ2)

^zzzzzz^

FIGURE 6.35

Redrawing an
intermediate cup

Force

then be annealed after every two operations in order to eliminate work-hardening and
thus avoid cracking and failure of the product.

Blank-development calculations. For the sake of simplicity, it is always assumed that
the thickness of the blank remains unchanged after the drawing operation. Because the
total volume of the metal is constant, it can then be concluded that the surface area of
the final product is equal to the surface area of the original blank. This rule forms the
basis for the blank-development calculations. Consider the simple example shown in
Figure 6.36. The surface area of the cup is the area of its bottom plus the area of the
wall:

surface area of cup = —d + ndh
4

FIGURE 6.36

A simple example of
blank development

VZZZ2ZZZZ0

-3

Surface area of blank -D2 = -d2 + ndh, i.e., surface area of the cup
4 4

230 6 Sheet Metal Working

This is equal to the surface area of the original blank; therefore, we can state that

—Dr = —d+ ndh

4 4

D2 = d2 + 4dh

Therefore, the original diameter of the blank, which is unknown, can be given by the
following equation:

D = Vd2 + 4dh

(6.6)

Equation 6.6 gives an approximate result because it assumes the cup has sharp corners,
which is not the case in industrial practice. However, this equation can be modified to
take round corners into account by adding the area of the surface of revolution result-
ing from the rotation of the round corner around the centerline of the cup, when equat-
ing the area of the product to that of the original blank. Note that the area of any
surface of revolution can be determined by employing Pappus's first theorem, which
gives that area as the product of the path of the center of gravity of the curve around
the axis of rotation multiplied by the length of that curve.

Planning for deep drawing. The process engineer usually receives a blueprint of the
required cup from the product designer. His or her job is to determine the dimensions
of the blank and the number of drawing operations needed, together with the dimen-
sions of intermediate cups, so that the tool designer can start designing the blanking
and the deep drawing dies. That job requires experience as well as close contact be-
tween the product designer and the process engineer. The following steps can be of
great help to beginners:

1. Allow for a small flange around the top of the cup after the operation is completed.
This flange is trimmed at a later stage and is referred to as the trimming allowance. It
is appropriate to take an allowance equal to 10 to 15 percent of the diameter of the cup.

2. Calculate the total surface area of the product and the trimming allowance. Then,
equate it to the area of the original blank with an unknown diameter. Next, solve
for the diameter of the original blank.

3. Calculate the thickness as a percentage of the diameter or (t/D) x 100, in order to
get a rough idea of the degree of wrinkling to be expected (see the preceding dis-
cussion on process variables).

4. Calculate the required percentage reduction. If it is less than or equal to 50, then the
required cup can be obtained in a single drawing. But if the required r is greater
than 50, then a few redrawing operations are required; the procedure to be followed
is given in the next steps.

5. For the first draw, assume r to be equal to 50 and calculate the dimensions of the
intermediate cup. Then, calculate r required for the first redraw. If r < 30, only a
single redraw is required.

6.1 Press Working Operations

231

6. If r > 30 for the first redraw, take it as equal to 30 and calculate the dimensions of
a second intermediate cup. The percentage reduction for the second redraw should
be less than 20; otherwise, a third redraw is required, and so on.

Ironing. We can see from the mechanics of the deep drawing operation that there is
reasonable variation in the thickness of the drawn cup. In most cases, such thickness
variation does not have any negative effect on the proper functioning of the product,
and, therefore, the drawn cups are used as is. However, close control of the dimensions
of the cups is sometimes necessary. In this case, cups are subjected to an ironing op-
eration, in which the wall of the cup is squeezed in the annular space between a punch
and its corresponding die. As can be seen in Figure 6.37, the punch-die clearance is
smaller than the thickness of the cup and is equal to the final required thickness. Large
reductions in thickness should be avoided in order to obtain a sound product. It is good
industrial practice to take the value of the punch-die clearance in the range between 30
and 80 percent of the thickness of the cup. Also, the percentage reduction in thickness,
which is given next, should fall between 40 and 60 in a single ironing operation. This
is a safeguard against fracture of the product during the operation. Following is the
equation to be applied:

percentage reduction in thickness =

tp-tf

x 100

(6.7)

where: tQ is the original thickness of the cup

tf is the final thickness of the cup after ironing

Drawing of stepped, conical, and domed cups. Stepped cups are those with two (or
more) shell diameters (see Figure 6.38a). They are produced in two (or more) stages.
First, a cup is drawn to have the large diameter, and, second, a redrawing operation is
performed on only the lower portion of the cup. Tapered or conical cups (see Figure

FIGURE 6.37

The ironing operation

FIGURE 6.38

Deep-drawn cups:

(a) stepped; (b) conical;

*ZZZZ\

&Z2Z&

Tvtv;

(a)

(b)

232

6 Sheet Metal Working

6.38b) cannot be drawn directly. They first have to be made into stepped cups, which
are then smoothed and stretched out to give the required tapered cups. A complex deep
drawing operation is used for producing domed cups (see Figure 6.38c). So that the
sheet metal stretches properly over the punch nose, higher blank-holding forces are re-
quired. Therefore, the process actually involves stretch-forming, and its variables
should be adjusted to eliminate either wrinkling or tearing.

Drawing of box-shaped cups. When all press working operations of sheet metal are
reviewed, there would be almost no doubt that the box drawing process is the most
complex and difficult to control. Nevertheless, in an attempt to simplify the problem,
we can divide a box into four round corners and four straight sides. Each of these
round corners represents 1/4 of a circular cup, and, therefore, the previous analysis
holds true for it. On the other hand, no lateral compression is needed to allow the blank
metal to flow toward the die edge at each of the straight sides. Accordingly, the process
in these zones is not drawing at all; it is just bending and straightening. For this rea-
son, the metal in these zones flows faster than in the round corners, and a square blank
takes the form shown in Figure 6.39 after drawing. Note that there is excess metal at
each of the four round corners, which impedes the drawing operations at those loca-
tions. It also results in localized higher stresses and tears almost always beginning at
one (or more) of the corners during box drawing, as can be seen in Figure 6.40.

Several variables affect this complex operation as well as the quality of the products
obtained. They include the die bending radius, the die corner radius, and the shape of the
original blank. These process variables have been investigated by research workers, and
it has been found that in order to obtain sound box-shaped cups, it is very important to
ensure easy, unobstructed flow of metal during the drawing operation. The absence of
this condition results in the initiation of high tensile stresses in the vertical walls of the
box, especially at the round corners, and results in considerable thinning, which is fol-
lowed by fracture. Among the factors that can cause obstruction to the metal flow are
smaller die radii, higher reduction ratios (at the corners), and poor lubrication. These are
added to the presence of excess metal at the corners, which causes an appreciable in-
crease in the transverse compressive stresses. Therefore, an optimum blank shape with-
out excess metal at the corners is necessary for achieving successful drawing operations
of box-shaped cups. A simple method for optimizing the shape of the blank is shown in
Figure 6.41. It involves printing a square grid on the surface of the blank and determin-
ing the borders of the undeformed zone on the flanges at each corner (by observing the

FIGURE 6.39

Final shape of a box-
shaped cup, obtained
by deep drawing a
square blank

6.1 Press Working Operations

233

FIGURE 6.40

Tears occurring in box
drawing

FIGURE 6.41

Optimized blank shape
for drawing box-shaped
cups

undistorted grid) so that it can be taken off the original blank. It has been found that the
optimum shape is a circle with four cuts corresponding to the four corners. Also, the
blank-holding force has been found to play a very important role. Better products are ob-
tained by using a rubber-actuated blank holder that exerts low forces during the first
third of the drawing stroke, followed by a marked increase in those forces during the rest
of the drawing stroke to eliminate wrinkling and stretch out the product.

234

Sheet Metal Working

FIGURE 6.42

Optimized blank shape
for drawing cups with
an irregular cross
section

Line

Original
blank

Cross section of the
deep-drawn part

The preceding discussion can be generalized to include the drawing of a cup with
an irregular cross section. This can be achieved by dividing the perimeter into straight
sides and circular arcs. Professor Kurt Lange and his coworkers (Institute Fur Um-
formstechnik, Stuttgart Universitate) have developed a technique for obtaining the op-
timum blank shape in this case by employing the slip-line field theory. The technique
was included in an interactive computer expert system that is capable of giving direct
answers to any drawing problem. An optimized blank shape obtained by that system is
shown in Figure 6.42.

Recent developments In deep drawing. A recent development in deep drawing in-
volves cup drawing without a blank holder. Cupping of a thick blank has been ac-
complished by pushing the blank through a die having a special profile, as shown in
Figure 6.43, without any need for a blank holder. This process has the advantages of
reducing the number of processing stages, eliminating the blank holder, and using
considerably simpler tool construction. A further advantage is that the operation can
be performed on a single-acting press, resulting in an appreciable reduction in the ini-
tial capital cost required.

Another new development is the employment of ultrasonics to aid the deep draw-
ing operation. The function of the ultrasonic waves is to enlarge the die bore and then
leave it to return elastically to its original dimension in a pulsating manner. This re-
duces the friction forces appreciably, resulting in a marked reduction in the required
drawing force and in a clear improvement of the quality of the drawn cup. In many
cases, the cup can be drawn by the force exerted by the human hand without the need

FIGURE 6.43

Drawing cups without a
blank holder

Blank

6.1 Press Working Operations

235

for any mechanical force-generating device. It is, therefore, obvious that low-tonnage,
high-production-rate presses can be used, which makes the process economically at-
tractive.

Defects in deep-drawn parts. These defects differ in shape and cause, depending
upon the prevailing conditions and also on the initial dimensions of the blank. Fol-
lowing is a brief description of the most common defects, some of which are shown in
Figure 6.44:

1. Wrinkling. Wrinkling is the buckling of the undrawn part of the blank under com-
pressive stresses; it may also occur in the vertical walls (see Figure 6.44a and b). If
it takes place on the punch nose when drawing a domed cup, it is referred to as
puckering.

2. Tearing. Tearing, which always occurs in the vicinity of the radius connecting the
cup bottom and the wall, is caused by high tensile stresses due to the obstruction of
the flow of the metal in the flange.

3. Earing. Earing is the formation of ears at the free edges of a deep-drawn cylindri-
cal cup (see Figure 6.44c). It is caused by the anisotropy of the sheet metal. Ears
are trimmed after a drawing operation, resulting in a waste of material.

4. Surface irregularities. Surface irregularities are caused by nonuniform yielding,
like the orange-peel effect of Luder's lines.

5. Surface marks. Surface marks are caused by improper punch-die clearance or poor
lubrication. These include draw marks, step rings, and burnishing.

Forming Operations

In this section, we will discuss the various forming operations performed on sheet met-
als— not just flat sheets, but tubular sheets (i.e., thin-walled tubes) as well. Therefore,
not only will operations like embossing and offsetting be discussed, but also tube
bulging, expanding, and necking will be considered.

Forming of sheets. True forming involves shaping the blank into a three-dimensional
(or sculptured) surface by sandwiching it between a punch and a die. The strain is not
uniform, and the operation is complex. The nonhomogeneity (or complexity) depends
upon the nature and the unevenness of the required shape. Experience and trial and
error were employed in the past to obtain an optimum blank shape and to avoid thin-
ning the blank or tearing.

FIGURE 6.44

Some defects occurring
in deep drawing
operations: (a) wrinkling
in the flange;

(b) wrinkling in the wall;

(b)

(c)

236

6 Sheet Metal Working

A printed grid on the original blank helps to detect the locations of overstraining
where tearing is expected. It also helps in optimizing the shape of the original blank.
With recent advances in computer graphics and simulation of metal deformation, ra-
tional design of the blank can be performed by the computer, without any need for trial
and error. In fact, a successful software package has been prepared by the Mechanical
Engineering Department of Michigan Technological University.

Embossing operations. Embossing operations involve localized deflection of a flat
sheet to create depressions in the form of beads and offsets. This is sometimes called
oil canning. Beads and offsets are usually employed to add stiffness to thin sheets,
whether flat or tubular (e.g., barrels), as well as for other functional reasons. A typi-
cal example of a part that is subjected to embossing is the license plate of an auto-
mobile. The cross section of a bead can take different forms, such as those shown in
Figure 6.45. Because this operation involves stretching the sheet, the achieved local-
ized percentage elongation within the bead cross section must be lower than that al-
lowable for the metal of the sheet. On the other hand, Figure 6.46 shows two kinds
of offsets, where it is common practice to take the maximum permissible depth as
three times the thickness of the sheet metal.

Rubber forming of flat sheets. Rubber forming is not new and actually dates back to
the nineteenth century, when a technique for shearing and cutting paper and foil was
patented by Adolph Delkescamp in 1872. Another rubber forming technique, called the
Guerin process, was widely used during World War II for forming aircraft panels. It in-
volved employing a confined rubber pad on the upper platen of the press and a steel
form block on the lower platen, as shown in Figure 6.47a. This method is still some-
times used. As can be seen in the figure, when a block of elastomer (usually incom-
pressible artificial rubber) is confined in a rigid box, the only way it can flow when the
punch sinks into it is up, thus forming the blank around the punch under uniform pres-
sure over the whole surface. It is also common industrial practice to place spacers on
the base of the metal box in order to provide a relief for the elastomer block, which, in
turn, helps to avoid the initiation of high localized strains in the blank area directly be-
neath the punch. Rubber forming has real potential when the number of parts required
is relatively small and does not justify designing and constructing a forming die.

A modified version of this process, called the hydroform process, involves em-
ploying a pressurized fluid above the rubber membrane, as shown in Figure 6.47b.

FIGURE 6.45

Different kinds of
beads

1 c

Vbead

Flat V bead

Round bead

FIGURE 6.46

Offsetting operations

Interior offset

Edge offset

6.1 Press Working Operations

237

FIGURE 6.47

Rubber forming of flat
sheets: (a) conventional
rubber forming;
(b) hydroform process

Rubber

Container

(a)

Rubber

Sheet
metal

Forming
punch

(b)

This is similar in effect to drawing the cup into a high-pressure container, as previously
mentioned. Therefore, percentage reductions higher than those obtained in conven-
tional deep drawing can be achieved.

Forming of tubular sheets. Figure 6.48a through d indicates tubular parts after they
were subjected to beading, flattening, expanding, and necking operations, respectively.
Tube bulging is another forming operation, in which the diameter of the tube, in
its middle part, is expanded and then restrained by a split die and forced to conform
to the details of the internal surface of the die. This can be achieved by internal hy-
draulic pressure or by employing an elastomer (polyurethane) rod as the pressure-
transmitting medium, causing expansion of the tube. A schematic of this operation is

FIGURE 6.48

Different tubular parts
after forming
operations: (a) beading;

(b) flattening;

(d) necking

(a)

(b)

-^7777.

\\\\\K

(d)

238

6 Sheet Metal Working

FIGURE 6.49

The tube-bulging
operation with an
elastomer rod

.Punch

Die holder
(ring)

Punch

given in Figure 6.49. At the beginning of the operation, the elastomer rod fits freely
inside the tube and has the same length. Compressive forces are then applied to both
the rod and the tube simultaneously so that the tube bulges outward in the middle
and the frictional forces at the tube-rod interface draw more metal into the die space,
thus decreasing the length of the tube. The method of using a polyurethane rod is
simpler and cleaner, and there is no need for using oil seals or complicated tooling
construction. A further advantage of rubber bulging is that it can be used for simul-
taneous forming, piercing, and shearing of thin tubular sheets.

6.2 HIGH-ENERGY-RATE FORMING (HERF)

In HERF, the energy of deformation is delivered within a very short period of time —
on the order of milliseconds or even microseconds. HERF methods include explosive,
electrohydraulic, and electromagnetic forming techniques. These techniques are usu-
ally employed when short-run products or large parts are required. HERF is also rec-
ommended for manufacturing prototype components and new shapes in order to avoid
the unjustifiable cost of dies. Rocket domes and other aerospace structural panels are
typical examples. During a HERF process, the sheet metal is given an extremely high
acceleration in a very short period of time and is thus formed as a result of consuming
its own kinetic energy to cause deformation.

Explosive Forming

Explosive forming of sheet metal received some attention during the past decade. The
various explosive forming techniques fall under one or the other of two basic systems:
confined and unconfined. In a confined system, which is shown in Figure 6.50a, a
charge of low explosives is detonated and yields a large amount of high-pressure gas,
thus forcing the sheet metal to take the desired shape. This system is mainly used for

6.2 High-Energy-Rate Forming (HERF)

239

FIGURE 6.50

Explosive forming of
sheet metal:

(a) confined system;

(b) standoff system

Cartridge

Water

Die steel

Tube

(b)

bulging and flaring of small tubular parts. Its main disadvantage is the hazard of die
failure because of the high pressure generated.

In an unconfined, or standoff, system, which is shown in Figure 6.50b, the charge is
maintained at a distance from the sheet blank (the standoff distance), and both the blank
and the charge are kept immersed in water. When the charge is detonated, shock waves
are generated, thus forming a large blank into the desired shape. It is obvious that the ef-
ficiency of the standoff system is less than that of the confined system because only a
portion of the surface over which the shock waves act is utilized (actually, shock waves
act in all directions, forming a spherical front). However, the standoff system has the ad-
vantages of a lower noise level and of largely reducing the hazard of damaging the
workpiece by particles resulting from the explosion. In a simple standoff system, the dis-
tance from the explosive charge to the water surface is usually taken as twice the stand-
off distance. The latter depends upon the size of the blank and is taken as equal to D (the
blank diameter) for D less than 2 feet (60 cm) and is taken as equal to 0.5D for D greater
than that. Best results are obtained when the blank is clamped lightly around its periph-
ery and when a material with a low modulus of elasticity, like plastic, is used as a die
material. This eliminates springback, thus obtaining closer tolerances. A modified ver-
sion of this method is illustrated in Figure 6.5 1, where a reflector is used to collect and

FIGURE 6.51

Increasing the
efficiency of explosive
forming by using a
reflector

Metal
blank

To vacuum

Explosive

Reflector

240

Sheet Metal Working

reflect the explosion energy that does not fall directly onto the blank surface. This leads
to improved efficiency over the standoff system because a smaller amount of charge is
needed for the same job.

Electrohydraulic Forming

The basic idea for the process of electrohydraulic forming, which has been known for
some time, is based on discharging a large amount of electrical energy across a small
gap between two electrodes immersed in water, as shown in Figure 6.52. The high-
amperage current resulting from suddenly discharging the electrical energy from the
condensers melts the thin wire between the electrodes and generates a shock wave.
The shock wave lasts for a few microseconds; it travels through water to hit the
blank and forces it to take the shape of the die cavity. The use of a thin wire between
the electrodes has the advantages of initiating and guiding the path of the spark, en-
abling the use of nonconductive liquids; also, the wire can be shaped to suit the
geometry of the required product. The method is also safer than explosive forming
and can be used for simultaneous operations like piercing and bulging. Nevertheless,
it is not suitable for continuous production runs because the wire has to be replaced
after each operation. Moreover, the level of energy generated is lower than that of ex-
plosive forming. Therefore, the products are generally smaller than those produced
by explosive forming.

Electromagnetic Forming

Electromagnetic forming is another technique based on the sudden discharge of elec-
trical energy. As we know from electricity and magnetism in physics, when an electric
current passes through a coil, it initiates a magnetic field whose magnitude is a func-
tion of the current. We also know that when a magnetic field is interrupted by a con-
ductive material (workpiece), a current is induced in that material that is proportional
to the rate of change of the flux. This is called eddy current and produces its own mag-
netic field that opposes the initial one. As a result, repulsive forces between the coil
and the workpiece force the workpiece to conform to the die cavity. This technique can
be used to form flat as well as tubular sheets. As can be seen in Figure 6.53, it is em-
ployed in expanding as well as compressing tubes. It has proven to be very effective
when forming relatively thin materials.

FIGURE 6.52

Electrohydraulic forming

Switch

Charger

IN-h. , r

HH HH HH

Capacitor
bank

Electrodes

6.3 Spinning of Sheet Metal

241

FIGURE 6.53

Examples of
electromagnetic
forming of tubes

Mandrel

PINNING OF SHEET METAL

Spinning is the forming of axisymmetric hollow shells over a rotating-form mandrel by
using special rollers. Generally, the shapes produced by spinning can also be manu-
factured by drawing, compressing, or flanging. However, spinning is usually used for
forming large parts that require very large drawing presses or when there is a diversity
in the products (i.e., when various shapes are needed but only a small number of each
shape is required).

A schematic of the spinning operation is shown in Figure 6.54. At the beginning,
the semifinished product (circular blank) is pushed by the tail stock against the front of
the form mandrel (usually a wooden one) that is fixed on the rotating faceplate of the
spinning machine (like a lathe). A pressing tool is pushed by the operator onto the ex-
ternal surface of the blank. The blank slips under the pressing devices, which causes
localized deformation. Finally, the blank takes the exact shape of the form mandrel.
This technique can also be used to obtain hollow products with a diameter at the end
(neck) smaller than that at the middle. In this case, it is necessary to use a collapsible-
form mandrel, which is composed of individual smaller parts that can be extracted
from the neck of the final product after the process is completed. Figure 6.55 shows a
group of parts produced by spinning.

FIGURE 6.54

A schematic of the
spinning operation

242

Sheet Metal Working

FIGURE 6.55

A group of parts
produced by spinning

A modified version of this method involves replacing the operator by a numeri-
cally controlled (NC) tool. Auxiliary operations, like removing the excess metal, are
also carried out on the same machine. Better surface quality and more uniform thick-
ness are the advantages of NC spinning over the conventional techniques.

Review Questions

1. What main design feature characterizes sheet
metal products?

2. List some of the advantages of press working
sheet metals.

3. When are sheet metals formed in their hot
state? Give examples.

4. What are the two main groups of press working
operations?

5. What main condition must be fulfilled so that
cutting of sheet metal (and not any other opera-
tion) takes place?

6. Use sketches to explain why the angle of incli-
nation of the upper blade of guillotine shears
must not exceed 15°.

7. Use sketches to differentiate between the fol-
lowing operations: shearing, cutoff, parting,
blanking, and punching.

8. Why must attention be given to careful layout
of blanks on a sheet metal strip?

9. Describe a perforating operation.

10. What does an edge of a blank usually look like?
Draw a sketch.

11. What is meant by the percentage penetration?

12. What does an edge of a blank look like when
the punch-die clearance is too large?

13. What does an edge of a blank look like when
the punch-die clearance is too tight?

Chapter 6 Review Questions

243

14. When are punches sheared and why?

15. When are dies sheared and why?

16. In what aspect is fine blanking different from
conventional blanking?

17. Use sketches to explain each of the following
operations: shaving, piercing, and cropping.

18. Can a drop-through die be used on any press?
Why not?

19. What is the function of a stripper plate?

20. List two types of die constructions for blanking
operations.

21. How can a washer be produced in a single
stroke?

22. What condition must be fulfilled so that bend-
ing of sheet metal takes place?

23. Sketch the common types of bending dies.

24. Which die requires the minimum force for the
same thickness of sheet metal?

25. Where is tearing expected to occur and where
is wrinkling expected to occur when a sheet
metal is subjected to bending?

26. What is springback? Why does it occur?

27. List three methods for eliminating the effects of
springback.

28. On what assumption is blank development
based?

29. List some operations that can be classified as
bending. Use sketches and explain design func-
tions of the products.

30. How can structural angles be bent?

31. Explain rotary bending, using sketches, and list
some of the advantages of this operation.

32. Explain how a seamed tube can be produced by
continuous bending.

33. What are the disadvantages of seamed tubes?

34. Explain deep drawing, using sketches.

35. What are the stages involved in deep drawing a
circular cup? Explain, using sketches.

36. Indicate the states of stress at different locations
in a cup toward the end of a drawing operation.

37. Where is thickening expected to occur?

38. At what location is thinning maximum? To
what would this lead?

39. Why is a blank holder sometimes needed?

40. List some of the variables affecting the deep
drawing operation.

41. What is wrinkling? Why does it occur?

42. Describe an ironing operation.

43. Is there any limitation on ironing?

44. Why are conical cups not drawn directly?

45. What is actually taking place when drawing
domed cups?

46. Is it feasible to take any blank shape for box
drawing operations and then trim the excess
metal? Why?

47. What are the mechanics of deformation in the
straight-sides areas?

48. What is the advantage of ultrasonic deep draw-
ing?

49. How can plates be drawn without a blank
holder?

50. List some of the defects experienced in deep-
drawn products.

51. As a product designer, how can you make use
of the embossing operation when designing
sheet metal parts?

52. When would you recommend using rubber
forming techniques?

53. What is meant by high-energy-rate forming?

54. When would you recommend using explosive
forming?

55. Should the dies used in explosive forming be
made of a hard material, like alloy steel, or a
softer one, like plastic? Why?

56. What happens if you make the hydraulic head
very small in explosive forming?

244

6 Sheet Metal Working

57. What are the advantages of electrohydraulic
forming? What are the disadvantages?

58. Use a sketch to explain the electromagnetic
forming operation.

59. Describe spinning. When is it recommended?

60. Can products with a diameter at the neck
smaller than at the middle be produced by spin-
ning? How?

Problems

1. The blank shown in Figure 6.56 is to be pro-
duced from a sheet metal strip 0.0625 inch (1.6
mm) in thickness. Material is low-carbon steel
AISI 1020. Estimate the required blanking force.

2. The products shown in Figure 6.57a, b, and c are
produced by bending. Obtain the length of the
original blank to the nearest 0.01 inch (0.25
mm). Take / as 0.0625 inch (1.6 mm).

3. A cup is drawn from a sheet of 1020 steel. The
thickness is 0.03 inch (0.8 mm), and the inner di-

ameter is 1 inch (25 mm). Estimate the maxi-
mum force required for drawing. If the material
is aluminum, what would the force be?

A cup with a height of 0.75 inch (18.75 mm) and
an inner diameter of 1 inch (25 mm) is to be
drawn from a steel strip 0.0625 inch ( 1 .6 mm) in
thickness. Plan for the drawing process by carry-
ing out blank development, determining the
number of drawings, and looking at the draw
severity analysis.

FIGURE 6.56

The blank shape
required in Problem 1

1.5 in.
(37.5 mm)

R = 0.25 in.
(6 mm)

Chapter 6 Design Example

245

FIGURE 6.57

Products produced by
bending in Problem 2

R = 0.5 in.
(12.5 mm)

R =0.75 in.

18.75 mm)

(c)

Design Example

PROBLEM

Design a simple wrench to loosen (or tighten) a 1/2-inch (12.5-mm) nut (or bolt head).
The 1/2 inch (12.5 mm) measures across the nut flats. The torque is 1 lb ft (1.356
Nm), and 50,000 pieces are required annually. The wrench is to be produced by press
working.

Solution

A suitable method for production is fine blanking as there will be no need for any fur-
ther machining operations. We cannot select a steel that has a high carbon content be-
cause it will create problems during the fine-blanking operation. An appropriate choice
is AISI 1035 CD steel. The dimensions of the wrench are the same as those given in
the examples on forging and casting, although the tolerances can be kept much tighter.
A detailed design is given in Figure 6.58.

246

Sheet Metal Working

FIGURE 6.58

Detailed design of a
wrench produced by
stamping

0.75 inch

Now it is time to check the maximum tensile stress due to bending:

/ = -±-bh3 = ^(0.25)(0.75)3 = 0.10546 x 10"2 in.4

q = Afr = l x 12 x 0.375 42851b/in2
/ 0.10546 x 10"2
It is less than the allowable stress for 1035 steel, which is about 20,000 lb/in.

Design Projects

1. A pulley (for a V-belt) that has 4-inch (100-mm) outer diameter and is mounted on
a shaft that is 3/4 inch (19 mm) in diameter was manufactured by casting. The
process was slow, and the rejects formed a noticeable percentage of the production.
As a product designer, you are required to redesign this pulley so that it can be pro-
duced by sheet metal working and welding.

2. Design a connecting rod for a sewing machine so that it can be produced by sheet
metal working, given that the diameter of each of the two holes is 0.5 inch (12.5
mm) and the distance between the centers of the holes is 4 inches (100 mm).

3. If a connecting rod four times smaller than the one of Design Project 2 is to be used
in a little toy, how would the design change?

4. Design a table for the machine shop. The table should be 4 feet in height, with a
surface area of 3 by 3 feet (900 by 900 mm), and should be able to carry a load

Chapter 6 Design Projects

247

of half a ton. Assume that 4000 pieces are required annually and that different
parts will be produced by sheet metal working and then joined together by nuts
and bolts.

5. A trash container having a capacity of 1 cubic foot (0.02833 m3) is to be designed
for manufacturing by sheet metal working. Assume that it is required to withstand
an axial compression load of 200 pounds (890 N) and that the production rate is
50,000 pieces per year. Provide a detailed design for this trash container.

6. A connecting lever is produced by forging. The lever has two short bosses, each at
one of its ends and each with a vertical hole that is 3/4 inch (19 mm) in diameter.
The horizontal distance between the centers of the holes is 12 inches (300 mm), and
the vertical distance is 3 inches (75 mm). The lever during functioning is subjected
to a bending moment of 200 lb ft (271 Nm). Because of the high percentage of re-
jects and low production rate, this connecting lever is to be produced by sheet metal
working. Provide a detailed design so that it can be produced by this manufactur-
ing method.

Chapter 7

wder —
tallurgy

INTRODUCTION

Powder metallurgy is the technology of producing useful components shaped
from metal powders by pressing and simultaneous or subsequent heating to
produce a coherent mass. The heating operation is usually performed in a con-
trolled-atmosphere furnace and is referred to as sintering. The sintering tem-
perature must be kept below the melting point of the powder material or the
melting point of the major constituent if a mixture of metal powders is used.
Therefore, sintering involves a solid-state diffusion process that allows the
compacted powder particles to bond together without going through the molten
state. This, in fact, is the fundamental principle of powder metallurgy.

Historical background. Although powder metallurgy is becoming increasingly im-
portant in modern industry, the basic techniques of this process are very old
indeed. The ancient Egyptians used a crude form of powder metallurgy as early
as 3000 b.c. to manufacture iron implements. The technique involved reducing
the ore with charcoal to obtain a spongy mass of metal that was formed by fre-
quent heating and hammering to eject the slag and consolidate the iron parti-
cles together into a mass of wrought iron. This process was used because the
primitive ovens then available were not capable of melting iron. The same tech-
nique was used later by smiths in India about a.d. 300 to manufacture the well-
known Delhi pillar weighing 6.5 tons. This method was superseded when more
advanced ovens capable of melting ferrous metals came into being.

At the beginning of the nineteenth century, powder metallurgy had its first
truly scientific enunciation, in England, when Wallaston published details of the

248

7.1 Metal Powders 249

preparation of malleable platinum. As had happened in the past, Wallaston's
technique was superseded by melting. However, the need for the powder met-
allurgy process arose again to satisfy the industrial demand for high-melting-
point metals. An important application was the production of ductile tungsten
in 1909 for manufacturing electric lamp filaments.

Why powder metallurgy? As a result of the development of furnaces and melt-
ing techniques, the powder-consolidation process is now usually used when
melting metal is undesirable or uneconomical. Fusion is not suitable when it is
required to produce parts with controlled, unique structures, such as porous
bearings, filters, metallic frictional materials, and cemented carbides. Also, it
has been found that powder metallurgy can produce certain complicated
shapes more economically and conveniently than other known manufacturing
processes. For this reason, the process currently enjoys widespread industrial
application. As the price of labor and the cost of materials continue to rise, the
powder-consolidation technique is becoming more and more economical be-
cause it eliminates the need for further machining operations, offers more ef-
ficient utilization of materials, and allows components to be produced in
massive numbers with good surface finish and close tolerances.

ETAL POWDERS

The Manufacture of Metal Powders

Different methods are used for producing metal powders. They include reduction of
metal oxides, atomization of molten metals, electrolytic deposition, thermal decompo-
sition of carbonyls, condensation of metal vapor, and mechanical processing of solid
metals.

Reduction. In reduction, the raw material is usually an oxide that is subjected to a se-
quence of concentration and purification operations before it is reduced. Carbon, car-
bon monoxide, and hydrogen are used as reducing agents. Following is the chemical
formula indicating the reaction between carbon and iron oxide:

2Fe,0, + 3C — > 4Fe + 3C02 T

(7.1)

Because the reaction takes place at a high temperature, the resulting metal particles
sinter together and form sponges that are subsequently crushed and milled to a powder
suitable for consolidation. Such powders have low apparent densities and often contain
impurities and inclusions, but they are cheap. Metal powders produced by this method
include iron, cobalt, nickel, tungsten, and molybdenum.

250

7 Powder Metallurgy

Atomization. Atomization is frequently used for producing powders from low-melt-
ing-point metals such as tin, lead, zinc, aluminum, and cadmium. Iron powder can also
be produced by atomization. The process involves forcing a molten metal through a
small orifice to yield a stream that is disintegrated by a jet of high-pressure fluid. When
compressed gas is used as the atomizing medium, the resulting powder particles will
be spherical. The reason is that complete solidification takes a relatively long period,
during which surface tension forces have the chance to spheroidize the molten metal
droplets. However, when water is used, the droplets solidify very quickly and have a
ragged or irregular form. Figure 7.1 illustrates the atomization technique.

Electrolytic deposition. Electrolytic deposition involves obtaining metal powders
from solutions by electrolysis. Process parameters such as current density and solution
concentration are controlled to give a loose deposit instead of the coherent layer ac-
quired in electroplating. The electrolytically deposited powders are then carefully

FIGURE 7.1

Production of metal
powders by atomization

Stream of
molten metal

Atomized
powder

7.1 Metal Powders 251

washed, dried, and annealed. Such powders are relatively expensive, but their impor-
tant advantage is their high purity and freedom from nonmetallic inclusions.

Thermal decomposition of carbonyls. Nickel and iron carbonyls are volatile liquids
having low boiling points of 110°F and 227°F (43°C and 107°C), respectively. They
decompose at temperatures below 572°F (300°C), and the metal is precipitated in the
form of a very fine powder.

Condensation of metal vapor. Condensation is employed only with some low-melt-
ing-point metals. For example, zinc powder can be obtained directly by condensation
of the zinc vapor.

Mechanical processing of solid metals. Production of metal powders by comminua-
tion of solid metals is accomplished by either machining, crushing, milling, or any
combination of these. This method is limited to the production of beryllium and mag-
nesium powders because of the expenses involved.

Properties of Metal Powders

The particular method used for producing a metal powder controls its particle and bulk
properties, which, in turn, affect the processing characteristics of that powder. There-
fore, comprehensive testing of all the physical and chemical properties of powders is
essential prior to use in order to avoid variations in the final properties of the com-
pacts. Following are the important characteristics of metal powders.

Chemical composition. In order to determine the chemical composition, conventional
chemical analysis is used in addition to some special tests that are applicable only to
metal powders, such as weight loss after reduction in a stream of hydrogen, which is
an indirect indication of the amount of oxide present. For example, in the case of iron
powder, the following equation is used:

159 7
% iron oxide = % weight loss x — — — (7.2)

= % weight loss x 3.33 (7.3)

In Equation 7.2, the fraction on the right-hand side is the ratio of the total weight of
iron oxide to the weight of the combined oxygen in it, or (Fe203)/(03), which can be
calculated by summing up the atomic weights of each element in the numerator and
denominator.

It is also important to mention that the percentages of nonmetallic inclusions will
affect the maximum achievable density of the compacted powder (i.e., the full theo-
retical density). For example, if an iron powder (density of iron is 7.87 g/cnv ) consists
of a percent Fe203, b percent carbon, and c percent sulfur, the following equation can
be applied:

100

max. achievable density = ioo - (a + b + c) a b c ^'^

+ + +

'•o/ Poxide Pcarbon Psulfur

252 7 Powder Metallurgy

where poxide, pcarbon> and Psuifur are the densities of oxide, carbon, and sulfur, respec-
tively. Equation 7.4 can also be used in calculating the maximum achievable density
for a mixture of powders.

Particle shape. The particle shape is influenced by the method of powder production
and significantly affects the apparent density of the powder, its pressing properties, and
its sintering ability.

Particle size. The flow properties and the apparent density of a metal powder are
markedly influenced by the particle size, which can be directly determined by mea-
surement on a microscope, by sieving, or by sedimentation tests.

Particle-size distribution. The particle-size distribution has a considerable effect on
the physical properties of the powder. Sieve testing is the standard method used for the
determination of the particle-size distribution in a quantitative manner. The apparatus
used involves a shaking machine on which a series of standard sieves are stacked with
the coarsest at the top and the finest at the bottom. The particle-size distribution is ob-
tained from the percentage (by weight) of the sample that passes through one sieve but
is retained on the next finer sieve. These sieves are defined by the mesh size, which in-
dicates the number of apertures per linear inch. After the test is performed, the results
are stated in a suitable form, such as a table of weight percentages, graphs of frequency
distribution, or cumulative oversize and undersize curves where the cumulative size is
the total weight percentage above or below a particular mesh size.

Specific surface. Specific surface is the total surface area of the particles per unit
weight of powder, usually expressed in square centimeters per gram (cm'/g). The spe-
cific surface has a considerable influence on the sintering process. The higher the spe-
cific surface, the higher the activity during sintering because the driving force for
bonding during the sintering operation is the excess energy due to the large area (high
specific surface).

Flowability. Flowability is the ease with which a powder will flow under gravity
through an orifice. A quantitative expression of the flowability of a powder is its flow
rate, which is determined using a Hall flowmeter. As illustrated in Figure 7.2, this ap-
paratus involves a polished conical funnel made of brass having a half-cone angle of
30° and an orifice of 0.125 inch (3.175 mm). The funnel is filled with 50 grams of the
powder, and the time taken for the powder to flow from the funnel is determined, the
flow rate being expressed in seconds. The flow properties are dependent mainly upon
the particle shape, particle size, and particle-size distribution. They are also affected by
the presence of lubricants and moisture. Good flow properties are required if high pro-
duction rates are to be achieved in pressing operations because the die is filled with
powder flowing under gravity and because a shorter die-filling time necessitates a high
powder-flow rate.

Bulk (or apparent) density. The bulk (or apparent) density is the density of the bulk
of a powder mass. It can be easily determined by filling a container of known volume
with the powder and then determining the weight of the powder. The bulk density is
the quotient of the powder mass divided by its volume and is usually expressed in
grams per cubic centimeter (g/cm3). The apparent density is influenced by the same

7.1 Metal Powders

253

FIGURE 7.2

A sketch of the Hall
flowmeter

1/8 in. or 1/10

factors as the flowability — namely, the particle configuration and the particle-size dis-
tribution.

Compressibility and compactibility. Compressibility and compactibility are very im-
portant terms that indicate and describe the behavior of a metal powder when com-
pacted in a die. Compressibility indicates the densification ability of a powder, whereas
compactibility is the structural stability of the produced as-pressed compact at a given
pressure. A generalized interpretation of these terms involves graphs indicating the as-
pressed density versus pressure (for compressibility) and the as-pressed strength ver-
sus pressure (for compactibility). It must be noted that these two terms are not
interchangeable: A brittle powder may have good compressibility but usually has a
weak as-pressed compactibility.

Sintering ability. Sintering ability is the ability of the adjacent surfaces of particles in
an as-pressed compact to bond together when heated during the sintering operation.
Sintering ability is influenced mainly by the specific surface of the powder used and is
the factor responsible for imparting strength to the compact.

Factors Affecting the Selection
of Metal Powders

Probably all metallic elements can be made in powderous form by the previously dis-
cussed manufacturing methods. However, the powder characteristics will differ in each
case and will depend mainly upon the method of manufacture. The task of the manu-
facturing engineer is to select the type of powder appropriate for the required job. The
decision generally depends upon the following factors:

1. Economic considerations

2. Purity demands

3. Desired physical, electrical, or magnetic characteristics of the compact

These considerations will be discussed in a later section.

254 7 Powder Metallurgy

7.2 POWDER METALLURGY:
THE BASIC PROCESS

The conventional powder metallurgy process normally consists of three operations:
powder blending and mixing, powder pressing, and compact sintering.

Blending and Mixing

Blending and mixing the powders properly is essential for uniformity of the finished
product. Desired particle-size distribution is obtained by blending in advance the types
of powders used. These can be either elemental powders, including alloying powders
to produce a homogeneous mixture of ingredients, or prealloyed powders. In both
cases, dry lubricants are added to the blending powders before mixing. The commonly
used lubricants include zinc stearate, lithium stearate, calcium stearate, stearic acid,
paraffin, acra wax, and molybdenum disulfide. The amount of lubricant added usually
ranges between 0.5 and 1.0 percent of the metal powder by weight. The function of the
lubricant is to minimize the die wear, to reduce the friction that is initiated between the
die surface and powder particles during the compaction operation, and, hence, to ob-
tain more even density distribution throughout the compact. Nevertheless, it is not rec-
ommended that the just-mentioned limits of the percentage of lubricant be exceeded,
as this will result in extruding the lubricant from the surfaces of the particles during
compaction to fill the voids, preventing proper densification of the powder particles
and impeding the compaction operation.

The time for mixing may vary from a few minutes to days, depending upon oper-
ator experience and the results desired. However, it is usually recommended that the
powders be mixed for 45 minutes to an hour. Overmixing should always be avoided
because it may decrease particle size and work-harden the particles.

Pressing

Pressing consists of filling a die cavity with a controlled amount of blended powder,
applying the required pressure, and then ejecting the as-pressed compact, usually
called the green compact, by the lower punch. The pressing operation is usually per-
formed at room temperature, with pressures ranging from 10 tons/in.2 (138 MPa) to
60 tons/in.^ (828 MPa), depending upon the material, the characteristics of the pow-
der used, and the density of the compact to be achieved.

Tooling is usually made of hardened, ground, and lapped tool steels. The final
hardness of the die walls that will come in contact with the powder particles during
compaction should be around 60 Rc in order to keep the die wear minimal. The die
cavity is designed to allow a powder fill about three times the volume (or height) of
the green compact. The ratio between the initial height of the loose powder fill and the
final height of the green compact is called the compression ratio and can be deter-
mined from the following equation:

7.2 Powder Metallurgy: The Basic Process

255

compression ratio

height of loose powder fill
height of green compact

density of green compact
apparent density of loose powder

(7.5)

When pressure is first applied to metal powders, they will undergo repacking or
restacking to reduce their bulk volume and to attain better packing density. The extent
to which this occurs depends largely on the physical characteristics of the powder par-
ticles. The movement of the powder particles relative to one another will cause the
oxide films covering their surfaces to be rubbed off. These oxide films will also col-
lapse at the initial areas of contact between particles because these areas are small and
the magnitude of the localized pressures are, therefore, extremely high. This leads to
metal-to-metal contact and, consequently, to cold-pressure welding between the pow-
der particles at the points of contact. When the pressure is further increased, interlock-
ing and plastic deformation of the particles take place, extending the areas of contact
between the individual particles and increasing the strength and density of the coher-
ent compacted powder. Plasticity of the metal-powder particles plays a major role dur-
ing the second stage of the pressing operation. As the compaction pressure increases,
further densification is increasingly retarded by work-hardening of the particle mater-
ial and by friction. Figure 7.3 shows a typical plot of the relationship between the
achieved density and the compaction pressure. As can be seen, the density first goes up
at a high rate, and then the rate of increase in density decreases with increasing pres-
sure. Consequently, it is very difficult to achieve the full density because prohibitive
pressure is required.

Frictional forces between the powder and the die wall always oppose the trans-
mission of the applied pressure in its vicinity. Therefore, the applied pressure diminishes
with depth in the case of single-ended pressing (i.e., when the compaction pressure is
applied on only one side). This is accompanied by an uneven density distribution
throughout the compact. The density always decreases with increasing distance from
the pressing punch face. Figure 7.4 indicates the variation of pressure with depth along

FIGURE 7.3

A typical plot of the
relationship between
achieved density and
compaction pressure

Compaction pressure

256

7 Powder Metallurgy

FIGURE 7.4

The variation of
pressure with depth
along the compact

the compact as well as the resulting variation in density. It is always recommended that
the value of the length-to-diameter ratio of the compact be kept lower than 2.0 in order
to avoid considerable density variations.

In order to improve pressure transmission and to obtain more even density distri-
bution, lubricants are either admixed with the powder or applied to the die walls. Other
techniques are also used to achieve uniform density distribution, such as compacting
from both ends and suspending the die on springs or withdrawing it to reduce the ef-
fects of die-wall friction.

During the pressing of a metal powder in a die, elastic deformation of the die oc-
curs in radial directions, leading to bulging of the die wall. Meanwhile, the compact
deforms both elastically and plastically. When the compaction pressure is released, the
elastic deformation tries to recover. But because some of the compact expansion is due
to plastic deformation, the die tightly grips the compact, which hinders the die from re-
turning to its original shape. Accordingly, a definite load, called the ejection load, has
to be exerted on the compact in order to push it out of the die. Figure 7.5 illustrates the
sequence of steps in a pressing operation.

Sintering

Sintering involves heating the green compact in a controlled-atmosphere furnace to a
temperature that is slightly below the melting point of the powder metal. When the
compact is composed of mixed elemental powders (e.g., iron and copper), the sinter-

FIGURE 7.5

Sequence of steps in a
pressing operation

W//,

v/////,

7.2 Powder Metallurgy: The Basic Process 257

ing temperature will then have to be below the melting point of at least one major con-
stituent. The sintering operation will result in the following:

1. Strong bonding between powder particles

2. Chemical, dimensional, or phase changes

3. Alloying, in the case of mixed elemental powders

Such effects of the sintering operation are influenced by process variables such as sin-
tering temperature, time, and atmosphere.

The amount, size, shape, and even nature of the pores are changed during sin-
tering. There are two kinds of porosity: open, or interconnected, porosity (connected
to the compact surface) and closed, or isolated, porosity. In a green compact, most
of the porosity is interconnected and is characterized by extremely irregular pores.
After sintering, interconnected porosity becomes isolated, and pore spheroidization
takes place because of the surface tension forces. Also, the oxide films covering the
particle surfaces of a green compact can be reduced by using the appropriate sin-
tering atmosphere.

The most important atmospheres used in industrial sintering are carbon monoxide,
hydrogen, and cracked ammonia. The latter is commonly used and is obtained by cat-
alytic dissociation of ammonia, which gives a gas consisting of 25 percent nitrogen
and 75 percent hydrogen by volume. Inert gases like argon and helium are occasion-
ally used as sintering atmospheres, but cost is a decisive factor in limiting their use.
Vacuum sintering is also finding some industrial application in recent years; neverthe-
less, the production rate is the main limitation of this method.

There are two main types of sintering furnaces: continuous and batch-operated.
In continuous furnaces, the charge is usually conveyed through the furnace on mesh
belts. These furnaces are made in the form of tunnels or long tubes having a diame-
ter of not more than 18 inches (45 cm). Heating elements are arranged to provide
two heating zones: a relatively low-temperature zone, called a dewaxing zone, in
which lubricants are removed so that they will not cause harmful reactions in the
next zone, and a uniform heating zone, which has the required high temperature
where sintering actually takes place. A third zone of the furnace tube is surrounded
by cooling coils in order to cool the compacts to ambient temperature in the con-
trolled atmosphere of the furnace, thus avoiding oxidation of the compacts. Flame
curtains (burning gases like hydrogen) are provided at both ends of the furnace tube
to prevent air from entering into the furnace. Figure 7.6 is a sketch of a continuous
sintering furnace. This type of furnace is suitable for mass production because of its
low sintering cost per piece and its ability to give more consistent products. When
small quantities of compacts must be sintered, however, batch-operated furnaces are
used. These furnaces (e.g., vacuum furnaces) are also more suitable when high-purity
products are required.

The sintering time varies with the metal powder and ranges between 30 minutes
and several hours. However, 40 minutes to an hour is the most commonly used sinter-
ing time in industry.

258

Powder Metallurgy

FIGURE 7.6

A sketch of a continuous sintering furnace

Temperature ,

£\

Flame
curtain

/@i

Uniform heating zone

Dewaxing
zone

Cooling
zone

Flame
curtain

W///////^^^^^

PERATIONAL FLOWCHART

Because of the wide variety of powder metallurgy operations, it may be difficult for
a person who is not familiar with this process to pursue the proper sequence of op-
erations. The flowchart in Figure 7.7 is intended to clearly show the relationship be-
tween the various powder metallurgy operations (which will be discussed later) and
to give a bird's-eye view of the flow of material to yield the final required product.
Nevertheless, it must be remembered that there are exceptions and that some opera-
tions cannot be shown on the flowchart because they would make it overly detailed
and complicated.

7.4 ALTERNATIVE CONSOLIDATION
TECHNIQUES

There are many techniques of consolidating metal powders. They are classified, as
shown in Figure 7.8, into two main groups: pressureless and pressure forming. The
pressureless methods are those in which no external pressure is required. This group
includes loose sintering, slip casting, and slurry casting. The, pressure forming methods
include conventional compaction, vibratory compaction, powder extrusion, powder

7.4 Alternative Consolidation Techniques

259

FIGURE 7.7

A flowchart showing the relationship between the various powder metallurgy operations

Metal powders

Mixing and blending

Conventional
die pressing

HERF
compaction

Cold
isostatic
pressing

Powder
rolling

Vibratory
compaction

Alternative

consolidation

technique

Slip
casting

Powder
extrusion

Hot
pressing

Hot
isostatic
pressing

Sintering

Secondary
processing

Finishing
operations

Finished P/M components

Loose
sintering

FIGURE 7.8

Classification of the techniques for consolidating metal powders

Consolidation
techniques

Pressureless

Pressure
forming

Loose sintering

Slip
casting

Conventional Vibratory
compaction compaction

Powder
extrusion

Powder
rolling

HIP

Slurry
casting

CIP Explosive Forming

compaction with binders

260 7 Powder Metallurgy

rolling, hot and cold isostatic pressing, explosive forming, and forming with binders.
A detailed account of conventional powder metallurgy has been given; following is a
brief description of these other consolidation techniques.

Loose Sintering

Loose sintering is employed in manufacturing filters. It involves sintering of loose
metal powder in molds made of graphite or ceramic material. The temperature used is
similar to that of conventional sintering, but the time involved is usually longer (two
days when manufacturing stainless steel filters).

Slip Casting

The application of slip casting is usually limited to the production of large, intricate
components made from refractory metals and cermets (mixtures of metals and ceram-
ics). The slip, which is a suspension of fine powder particles in a viscous liquid, is
poured into an absorbent plaster-of-paris mold. Both solid and hollow articles can be
produced by this method. When making hollow objects, excess slip is poured out after
a layer of metal has been formed on the mold surface.

Slurry Casting

Slurry casting is very similar to slip casting, except that the mixture takes the form of
a slurry and binders are usually added. Also, because the slurry contains less water,
nonabsorbent molds can be used.

Vibratory Compaction

Vibratory compaction involves superimposing mechanical vibration on the pressing
load during the compaction operation. The advantages of this process include the con-
siderable reduction in the pressure required and the ability to compact brittle particles
that cannot be pressed by conventional techniques because the high compaction load
required would result in fragmentation rather than consolidation of the powder parti-
cles. The main application involves the consolidation of stainless steel and uranium
oxide powders for nuclear fuel elements.

Isostatic Pressing

In isostatic pressing (IP), equal all-around pressure is applied directly to the powder
mass via a pressurized fluid. Accordingly, die-wall friction is completely eliminated,
which explains the potential of the process to produce large, dense parts having uni-
form density distribution. The process can be performed at room temperature (cold iso-
static pressing) or can be carried out at elevated temperatures (hot isostatic pressing).
In cold isostatic pressing (CIP), a flexible envelope (usually made of rubber or
polymers) that has the required shape is filled with the packed powder. The envelope
is then sealed and placed into a chamber that is, in turn, closed and pressurized to con-

7.4 Alternative Consolidation Techniques

261

FIGURE 7.9

The isostatic pressing
operation

Pressurized
fluid

esS9

Threaded
plug

o o o
o o o

solidate the powder. The lack of rigidity of the flexible envelope is countered by using
a mesh or perforated container as a support (see Figure 7.9). The main disadvantage of
this process is the low dimensional accuracy due to the flexibility of the mold.

In hot isostatic pressing (HIP), both isostatic pressing and sintering are combined.
Powder is canned in order to separate it from the pressurized fluid, which is usually
argon. The can is then heated in an autoclave, with pressure applied isostatically. Com-
plete densification and particle bonding occur. The elevated temperature at which the
powder is consolidated results in a softening of the particles. For this reason, the
process is used to compact hard-to-work materials such as tool steels, beryllium,
nickel-base superalloys, and refractory metals. A good example is the manufacture of
jet-engine turbine blades, where a near-net shape is made from nickel-base superal-
loys. A main disadvantage of this method is the long processing time.

Powder Extrusion

Powder extrusion is a continuous compaction process and can be performed hot or
cold. It is employed in producing semifinished products having a high length-to-
diameter ratio, a geometry that makes producing them by conventional powder met-
allurgy impossible. The conventional technique involves packing metal powder into
a thin container that is, in turn, evacuated, sealed, and then extruded. An emerging
technique involves the extrusion of suitable mixtures of metal (or ceramic) powders
and binders such as dextrin and sugars. It has been successfully employed in the
production of highly porous materials used as filters or fuel cells in batteries.

262 7 Powder Metallurgy

Powder Rolling

Direct powder rolling, or roll compacting, is another type of continuous compaction
process. It is employed mainly for producing porous sheets of nonferrous powders like
copper and nickel. This process involves feeding the metal powder into the gap be-
tween the two rolls of a simple mill, where it is squeezed and pushed forward to form
a sheet that is sintered and further rolled to control its density and thickness.

High-Energy-Rate Compaction

The various HERF compaction techniques are based on the same principle, which is
the application of the compaction energy within an extremely short period of time.
Several methods were developed for compacting metal powders at high speeds. Ex-
amples are explosives, high-speed presses, and spark sintering. It is believed that ex-
plosive compaction is suitable only when the size of the compact and the density
required cannot be achieved by the isostatic compaction process. Nevertheless, the
danger of handling explosives and the low cycling times impose serious limitations on
this technique in production.

The use of high-speed presses like the Dynapak (built by General Dynamics)
and the Petro-Forge (built by Mechanical Engineering Department, Birmingham
University, England) for powder compaction is, in practicality, an extension of the
die-pressing technique. These high-speed presses are particularly advantageous for
pressing hard-to-compact powders and large components.

There are also some other powder-consolidation methods that can be classified as
high-speed techniques. These include electrodynamic pressing, electromagnetic press-
ing, and spark sintering. Electrodynamic pressing involves utilizing the high pressure
produced by the sudden discharge of electrical energy to compact powders at high
speeds. Electromagnetic pressing is based upon the phenomenon that a strong mag-
netic field is generated when electric current is suddenly discharged through an in-
ductance. This strong magnetic field is used for pressing a thin-walled metallic tube
that contains the powder. Spark sintering involves the sudden discharge of electrical
energy into the powder mass to puncture the oxide films that cover each individual
powder particle and to build up pure metallic contacts between the particles. After
about 10 seconds of impulsive discharging, the current is shut off, and a pressure of
about 14,500 lb/in2 (100 MPa) is applied to compact the powder to the final required
form.

Injection Molding

Although injection molding is an emerging process, it can be considered as a version
of forming with binders, which is a rather old method. The process involves injection
molding metal powders that are precoated with a thermoplastic polymer into a part
similar in shape to the final required component but having larger dimensions. After re-
moving the polymer by an organic solvent, the porous compact is then sintered for a
long time in order to allow for volume shrinkage and, consequently, an increase in
density. The main advantage of this process is that it offers promise in the forming of
intricate shapes.

7.5 Secondary Consolidation Operations 263

Hot Pressing

Hot pressing is a combination of both the compaction and the sintering operations. It
is basically similar to the conventional powder metallurgy process, except that pow-
ders are induction heated during pressing, and, consequently, a protective atmosphere
is necessary. For most metal powders, the temperatures used are moderate (above re-
crystallization temperature), and dies made of superalloys are used. The hot pressing
of refractory metals (e.g., tungsten and beryllium), however, necessitates the use of
graphite dies. The difficulties encountered in this technique limit its application to lab-
oratory research.

SECONDARY CONSOLIDATION
OPERATIONS

In most engineering applications, the physical and mechanical properties of the as-
sintered compact are adequate enough to make it ready for use. However, secondary
processing is sometimes required to increase the density and enhance the mechani-
cal properties of the sintered component, thus making it suitable for heavy-duty en-
gineering applications. The operations involved are similar to those used in forming
fully dense metals, though certain precautions are required to account for the porous
nature of the sintered compacts. Following is a survey of the common secondary
operations.

Coining (Repressing)

Coining involves the pressing of a previously consolidated and sintered compact in
order to increase its density. This operation is performed at room temperature, and con-
siderable pressures are thus required. It is often possible to obtain significant improve-
ment in strength not only because of the increased densification but also because of the
work-hardening that occurs during the operation. A further advantage of this process is
that it can be employed to alter shape and dimensions slightly. Repressing is a special
case of coining where no shape alteration is required.

Extrusion, Swaging, or Rolling

Sintered powder compacts, whether in their cold or hot state, can be subjected to any
forming operation (extrusion, swaging, or rolling). When processing at elevated tem-
peratures, either a protective atmosphere or canning of the compacts has to be em-
ployed. Such techniques are applied to canned sintered compacts of refractory metals,
beryllium, and composite materials.

Forging of Powder Preforms

Repressing and coining of sintered compacts cannot reduce porosity below 5 percent
of the volume of the compact. Therefore, if porosity is to be completely eliminated,
hot forging of powder preforms must be employed. Sintered powder compacts hav-
ing medium densities (80 to 85 percent of the full theoretical density) are heated.

264

Powder Metallurgy

lubricated, and fed into a die cavity. The preform is then forged with a single
stroke, as opposed to conventional forging of fully dense materials, where several
blows and manual transfer of a billet through a series of dies are required. This ad-
vantage is a consequence of using a preform that has a shape quite close to that of
the final forged product. The tooling used involves a precision flashless closed die;
therefore, the trimming operation performed after conventional forging is eliminated.
The forging of powder preforms combines the advantages of both the basic pow-
der metallurgy and the conventional hot forging processes while eliminating their
shortcomings. For this reason, the process is extensively used in the automotive in-
dustry in producing transmission and differential-gear components. Examples of some
forged powder metallurgy parts are shown in Figure 7.10.

7.6 FINISHING OPERATIONS

Many powder metallurgy products are ready for use in their as-sintered state; however,
finishing processes are frequently used to impart some physical properties or geomet-
rical characteristics to them. Following are some examples of the finishing operations
employed in the powder metallurgy industry.

Sizing

Sizing is the pressing of a sintered compact at room temperature to secure the desired
shape and dimensions by correcting distortion and change in dimensions that may have
occurred during the sintering operation. Consequently, this operation involves only

FIGURE 7.10

Some forged powder
metallurgy parts
(Courtesy of the Metal
Powder Industries
Federation, Princeton,
New Jersey)

(a)

7.6 Finishing Operations

265

FIGURE 7.10
(Cont.)

Some forged powder
metallurgy parts
(Courtesy of the Metal
Powder Industries
Federation, Princeton,
New Jersey)

(b)

limited deformation and slight density changes and has almost no effect on the me-
chanical properties of the sintered compact.

Machining

Features like side holes, slots, or grooves cannot be formed during pressing, and, there-
fore, either one or two machining operations are required. Because cooling liquids
can be retained in the pores, sintered components should be machined dry whenever
possible. An air blast is usually used instead of coolants to remove chips and cool the
tool.

266 7 Powder Metallurgy

Oil Impregnation

Oil impregnation serves to provide either protection against corrosion or a degree of
self-lubrication or both. It is usually carried out by immersing the sintered porous com-
pact in hot oil and then allowing the oil to cool. Oil impregnation is mainly used in the
manufacturing of self-lubricating bearings made of bronze or iron.

Infiltration

Infiltration is permeation of a porous metal skeleton with a molten metal of a lower
melting point by capillary action. Infiltration is performed in order to fill the pores and
give two-phase structures with better mechanical properties. The widely used applica-
tion of this process is the infiltration of porous iron compacts with copper. The process
is then referred to as copper infiltration and involves placing a green compact of cop-
per under (or above) the sintered iron compact and heating them to a temperature
above the melting point of copper.

Heat Treatment

Conventional heat treatment operations can be applied to sintered porous materials,
provided that the inherent porosity is taken into consideration. Pores reduce the ther-
mal conductivity of the porous parts and thus reduce their rate of cooling. For sintered
porous steels, this means poorer hardenability. Also, cyanide salts, which are very poi-
sonous and are used in heat treatment salt baths, are retained in the pores, resulting in
extreme hazards when using such heat-treated compacts. Therefore, it is not advisable
to use salt baths for surface treatment of porous materials.

Steam Oxidizing

A protective layer of magnetite (Fe304) can be achieved by heating the sintered ferrous
parts and exposing them to superheated steam. This will increase the corrosion resis-
tance of the powder metallurgy parts, especially if it is followed by oil impregnation.

Plating

Metallic coatings can be satisfactorily electroplated directly onto high-density and
copper-infiltrated sintered compacts. For relatively low-density compacts, electro-
plating must be preceded by an operation to seal the pores and render the compacts
suitable for electroplating.

7.7 POROSITY IN POWDER
METALLURGY PARTS

The structure of a powder metallurgy part consists of a matrix material with a mi-
crostructure identical to that of a conventional fully dense metal and pores that are a
unique and controllable feature of sintered porous materials. For this reason, powder

7.7 Porosity in Powder Metallurgy Parts

267

metallurgy materials are grouped according to their porosity, which is quantitatively
expressed as the percentage of voids in a part. Those materials having less than 10 per-
cent porosity are considered to be high density; those with porosity more than 25 per-
cent, low density. There is a relationship between porosity and density (both being
expressed as fractions of the full theoretical density), and it can be expressed by the
following equation:

porosity = 1 - density (7.6)

As previously explained, the theoretical density is not that of the fully dense pure
metal but is the mean value of the densities of all constituents. These include not only
alloying additives but also impurities. When considering green densities, the effect of
lubricants must be taken into consideration.

Pores are classified with respect to their percentage, type, size, shape, and distrib-
ution. The type can be either interconnected or isolated. The volume of interconnected
porosity can be determined by measuring the amount of a known liquid needed to sat-
urate the porous powder metallurgy sample. The interconnected porosity is essential
for successful oil impregnation and thus is very important for the proper functioning
of self-lubricating bearings.

At this stage, it is appropriate to differentiate between the following three techni-
cal terms used to describe density:

„ , . mass of compact n -,

bulk density = — ; j^ VJ>

bulk volume of compact

mass of compact

apparent density = ;

rr apparent volume

(/.©)

mass of compact

bulk volume of compact - volume of open pores
mass of compact

true density =

true volume

mass of compact "™

~ bulk volume of compact - (volume of open pores
+ volume of closed pores)

For a green compact produced by admixed lubrication, these densities are mis-
leading and do not indicate the true state of densification due to the presence of lubri-
cant within the space between metal particles. Therefore, the bulk density must be
readjusted to give the true metal density (TMD) as follows:

, , „ , % of metal n im

TMD = actual bulk density x — I7-™)

268 7 Powder Metallurgy

ESIGN CONSIDERATIONS FOR POWDER
METALLURGY PARTS

The design of a powder metallurgy part and the design of the tooling required to pro-
duce it cannot be separated. A part design that needs either long, thin tubular punches,
tooling with sharp corners, or lateral movement of punches cannot be executed. For
this reason, the design of powder metallurgy parts is often different from that of parts
produced by machining, casting, or forging, and a component that is being produced
by these methods has to be redesigned before being considered for manufacture by
powder metallurgy. Following are various tooling and pressing considerations, some of
which are illustrated in Figure 7.11.

Holes

Holes in the pressing direction can be produced by using core rods. In this case, there
is almost no limitation on the general shape of the hole. But side holes and side slots
are very difficult to achieve during pressing and must be made by secondary machin-
ing operations (see Figure 7.11a).

Wall Thickness

It is not desirable to have a wall thickness less than 1/16 inch (1.6 mm) because the
punch required to produce the thickness will not be rigid enough to withstand the high
stresses encountered during the pressing operation.

Fillets

It is recommended that sharp corners be avoided whenever possible. Fillets with gen-
erous radii are desirable, provided that they do not necessitate the use of punches with
featherlike edges (see Figure 7.11b).

Tapers

Tapers are not always required. However, it is desirable to have them on flange-type
sections and bosses to facilitate the ejection of the green compact.

Chamfers

As mentioned earlier, it is sometimes not desirable to use radii on part edges. Cham-
fers are the proper alternative in preventing burrs.

Flanges

A small flange, or overhang, can be easily produced. However, for a large overhang,
ejection without breaking the flange is very difficult (see Figure 7.11c).

7.8 Design Considerations for Powder Metallurgy Parts

269

FIGURE 7.11

Design considerations
for powder metallurgy
parts: (a) holes;
(b) fillets; (c) flanges;

(d) bosses;

(e) undercuts

Required ^
punch

(b)

(c)

\ /

(d)

(e)

Bosses

Bosses can be made, provided that they are round in shape (or almost round) and that
the height does not exceed 15 percent of the overall height of the component (see
Figure 7. lid).

Undercuts

Undercuts that are perpendicular to the pressing direction cannot be made because they
prevent ejection of the part. If required, they can be produced by a secondary machin-
ing operation (see Figure 7.1 le).

270 7 Powder Metallurgy

7.9 ADVANTAGES AND DISADVANTAGES
OF POWDER METALLURGY

Like any other manufacturing process, powder metallurgy has advantages as well as
disadvantages. The decision about whether to use this process or not must be based on
these factors. The advantages of powder metallurgy are as follows:

1. Components can be produced with good surface finish and close tolerances.

2. There is usually no need for subsequent machining or finishing operations.

3. The process offers a high efficiency of material utilization because it virtually
eliminates scrap loss.

4. Because all steps of the process are simple and can be automated, only a mini-
mum of skilled labor is required.

5. Massive numbers of components with intricate shapes can be produced at high
rates.

6. The possibility exists for producing controlled, unique structures that cannot be
obtained by any other process.

The main disadvantages of the process are as follows:

1. Powders are relatively high in cost compared with solid metals.

2. Sintering furnaces and special presses, which are more complicated in principle
and construction than conventional presses, are necessary.

3. Tooling is very expensive as several punches or die movements are often used.

4. High initial capital cost is involved, and the process is generally uneconomical
unless very large numbers of components are to be manufactured.

5. Powder metallurgy parts have inferior mechanical properties due to porosity (this
does not apply to forged powder metallurgy parts), and the process is thus primar-
ily suitable for the production of a large number of small, lightly stressed parts.

7.10 APPLICATIONS OF POWDER
METALLURGY PARTS

The applications of powder metallurgy parts fall into two main groups. The first group
consists of those applications in which the part is used as a structural component that
can also be produced by alternative competing manufacturing methods, powder metal-
lurgy being used because of the low manufacture cost and high production rate. The
second group includes those applications in which the part usually has a controlled,
unique structure and cannot be made by any other manufacturing method. Examples
are porous bearings, filters, and composite materials. Following is a quick review of
the various applications.

7.10 Applications of Powder Metallurgy Parts

271

FIGURE 7.12

Some powder
metallurgy products
(Courtesy of the Metal
Powder Industries
Federation, Princeton,
New Jersey)

Structural Components

Powder metallurgy used to be limited to the production of small, lightly stressed parts.
However, with the recent development in forging powder preforms, the process is
commonly used in producing high-density components with superior mechanical prop-
erties. Cams, gears, and structural parts of the transmission system are some applica-
tions of the powder metallurgy process in the automotive, agricultural machinery, and
domestic appliance industries. Figures 7.12 and 7.13 show some examples of powder
metallurgy products.

The structural powder metallurgy components are usually made of iron-base pow-
ders, with or without additions of carbon, copper, and other alloying elements like

FIGURE 7.13

More powder metallurgy
products (Courtesy of
the Metal Powder
Industries Federation,
Princeton, New Jersey)

272 7 Powder Metallurgy

nickel. Prealloyed powders are also employed, although they are less common than the
mixed elemental powders.

Self-Lubricating Bearings

Self-lubricating bearings are usually made by the conventional die-pressing technique,
in which a porosity level between 20 and 40 percent is achieved. A sizing operation is
performed for dimensional accuracy and in order to obtain smooth surfaces. The bear-
ings are oil impregnated either before or after sizing. Bronze powders are used in the
manufacturing of porous bearings, but iron-base powders are also employed to give
higher strength and hardness.

Filters

In manufacturing filters, the appropriate metal powder (e.g., bronze) is screened in
order to obtain uniform particle size. The powder is then poured into a ceramic or
graphite mold. The mold is put into a sintering furnace at the appropriate sintering
temperature so that loose sintering can take place. The products must have generous
tolerances, especially on their outer diameters, where 3 percent is typical.

Friction Materials

Clutch liners and brake bands are examples of friction materials. They are best manu-
factured by powder metallurgy. The composition includes copper as a matrix, with ad-
ditions of tin, zinc, lead, and iron. Nonmetallic constituents like graphite, silica, emery,
or asbestos are also added. The mixture is then formed to shape by cold pressing. After
sintering, some finishing operations like bending, drilling, and cutting are usually re-
quired. It must be noted that friction materials are always joined to a solid plate, which
gives adequate support to these weak parts.

Electrical Contact Materials

Electrical contact materials include two main kinds: sliding contacts and switching
contacts. It is not possible to produce any of these contact materials except by powder
metallurgy as both involve duplex structures.

Sliding contacts are components of electrical machinery employed when current is
transferred between sliding parts (e.g., brushes in electric motors). The two main char-
acteristics needed are a low coefficient of friction and good electrical conductivity.
Compacts of mixtures of graphite and metal powder can fulfill such conditions. Pow-
ders of metals having high electrical conductivity, such as brass, copper, or silver, are
used. These graphite-metal contacts are produced by conventional pressing and sinter-
ing processes.

Switching contacts are used in high-power circuit breakers. The three characteris-
tics needed are good electrical conductivity, resistance to mechanical wear, and less
tendency of the contact surfaces to weld together. A combination of copper, silver, and
a refractory metal like tungsten provides the required characteristics. These contacts

7.10 Applications of Powder Metallurgy Parts 273

are produced either by conventional pressing and sintering or by infiltrating a porous
refractory material with molten copper or silver.

Magnets

Magnets include soft magnets and permanent magnets. Soft magnets are used in dc
motors or generators as armatures, as well as in measuring instruments. They are made
of iron, iron-silicon, and iron-nickel alloys. Electrolytic iron powder is usually used
because of its high purity and its good compressibility, which allows the high compact
densities required for maximum permeability to be attained.

Permanent magnets produced by powder metallurgy have the commonly known
name Alnico. This alloy consists mainly of nickel (30 percent), aluminum (12 percent),
and iron (58 percent) and possesses outstanding permanent magnetic properties. Some
other additives are often used, including cobalt, copper, titanium, and niobium.

Cores

The cores produced by powder metallurgy are used with ac high-frequency inductors
in wireless communication systems. Such cores must possess high constant perme-
ability for various frequencies as well as high electrical resistivity. Carbonyl iron pow-
der is mixed with a binder containing insulators (to insulate the powder particles from
one another and thus increase electrical resistivity) and then compacted using ex-
tremely high pressures, followed by sintering.

Powder Metallurgy Tool Steels

The production of tool steels by powder metallurgy eliminates the defects encountered
in conventionally produced tool steels — namely, segregation and uneven distribution
of carbides. Such defects create problems during tool fabrication and result in shorter
tool life. The technique used involves compacting prealloyed tool-steel powders by hot
isostatic pressing to obtain preforms that are further processed by hot working.

Superalloys

Superalloys are nickel- and cobalt-base alloys, which exhibit high strength at elevated
temperatures. They are advantageous in manufacturing jet-engine parts like turbine
blades. The techniques used in consolidating these powders include HIP, hot extrusion,
and powder metallurgical forging.

Refractory Metals

The word refractory means "difficult to fuse." Therefore, metals with high melting
points are considered to be refractory metals. These basically include four metals:
tungsten, molybdenum, tantalum, and niobium. Some other metals can also be con-
sidered to belong to this group. Examples are platinum, zirconium, thorium, and tita-
nium. Refractory metals, as well as their alloys, are best fabricated by powder
metallurgy. The technique used usually involves pressing and sintering, followed by

274

Powder Metallurgy

working at high temperatures. The applications are not limited to incandescent lamp
filaments and heating elements; they also include space technology materials, the
heavy metal used in radioactive shielding, and cores for armor-piercing projectiles.
Titanium is gaining an expanding role in the aerospace industry because of its excel-
lent strength-to-specific-weight ratio and its good fatigue and corrosion resistance.

Cemented Carbides

Cemented carbides are typical composite materials that possess the superior properties
of both constituents. Cemented carbides consist of hard wear-resistant particles of
tungsten or titanium carbides embedded in a tough strong matrix of cobalt or steel.
They are mainly used as cutting and forming tools; however, there are other applica-
tions, including gages, guides, rock drills, and armor-piercing projectiles. They possess
excellent red hardness and have an extremely long service life as tools. Cemented car-
bides are manufactured by ball-milling carbides with fine cobalt (or iron) powder, fol-
lowed by mixing with a lubricant and pressing. The green compact is then presintered
at a low temperature, machined to the required shape, and sintered at an elevated tem-
perature. A new dimension in cemented carbides is Ferro-Tic, involving titanium car-
bide particles embedded in a steel matrix. This material can be heat treated and thus
can be easily machined or shaped.

Review Questions

1. Define each of the following technical terms:

a. compressibility

b. compactibility

c. green density

d. impregnation

e. infiltration

f. flowability

g. particle-size distribution

2. List five advantages of the powder metallurgy
process.

3. List four disadvantages of the powder metal-
lurgy process.

4. What are the important characteristics of a
metal powder?

5. Describe three methods for producing metal
powders.

Explain briefly the mechanics of pressing.

Why are lubricants added to metal powders be-
fore pressing?

Is it possible to eliminate all voids by conven-
tional die pressing? Why?

9. Explain briefly the mechanics of sintering.

10. Why is it necessary to have controlled atmos-
pheres for sintering furnaces?

Explain why it is not possible to use the con-
ventional pressing techniques as a substitute for
each of the following operations: isostatic
pressing, slip casting, HERF compaction.

Differentiate between the following: coining,
repressing, sizing.

How is copper infiltration accomplished and
what are its advantages?

Chapter 7 Problems

275

14. Can powder metallurgical forging be replaced
by conventional forging? Why?

15. How can machining of some powder metal-
lurgy components be inevitable?

16. How is plating of powder metallurgy compo-
nents carried out?

17. Name five products that can only be produced
by powder metallurgy.

18. Why are cemented carbides presintered?

19. Why is electrolytic iron powder used in manu-
facturing soft magnets?

20. Discuss four design limitations in connection
with powder metallurgy components.

Problems

1. Following are the experimentally determined
characteristics of three kinds of iron powder:

Sponge Iron Powder (1)

Apparent

Screen Analysis

Chemical

Composition

Density

Row Rate

+70 0%

H2 loss

0.2%

-70 to +100 1%

0.02%

2.4 g/cm3

35 s/50 g

-100 to +325 74%

Si02

0.2%

-325 25%

P
S

0.015%
0.015%

Sponge Iron Powder (II)

Apparent

Screen Analysis

Chemical

Composition

Density

Flow Rate

+40 2%

H2 loss

0.2%

-40 to +60 40%

0.1%

2.4 g/cm3

35 s/50 g

-70 to +100 30%

Si02

0.3%

-100 to +200 20%

0.015%

-200 8%

0.015%

276

7 Powder Metallurgy

Atomized Iron Powder

Screen Analysis

Chemical

Composition

Apparent
Density

Flow Rate

+100

H2 loss

0.2%

+140

45%

0.01%

2.9 g/cm3

24 s/50 g

+200

25%

+230

15%

+325

10%

-325

Plot the following for each powder:

a. Cumulative oversize graph

b. Cumulative undersize graph

c. Frequency distribution curve (obtain median
particle size for the powder)

d. Histogram of particle-size distribution

The axis usually indicates the particle size in mi-
crons and not mesh size. Use the following
table:

4. Plot a graph indicating the maximum achievable
green density versus the percentage of admixed
zinc stearate for atomized iron powder (density
of zinc stearate is 1 . 1 g/cm ). What can you de-
duce from the curve?

5. Which powder in Problem 1 would fill the die
cavity faster?

Sieve
Microns

420

60
250

70
210

100
149

240
105

200

230
63

325

Calculate the full theoretical density for a com-
pact made of atomized iron powder, knowing
that the density of carbon equals 2.2 g/cm and
the density of iron oxide equals 2.9 g/cm .

Determine the approximate height of the pow-
der fill for each kind of iron powder given in
Problem 1 if the green density of the compact is
6.8 g/cm3 and its height is 2.1 cm.

Calculate the maximum achievable green density
of a mixture of atomized iron powder plus 1 per-
cent zinc stearate and 10 percent pure copper
(density of copper is 8.9 g/cm3).

Chapter 7 Design Project

277

7. Following is the relationship between density
and pressure for atomized iron powder contain-
ing 1 percent zinc stearate:

8. A cylindrical compact of atomized iron powder
plus 1 percent zinc stearate had a green bulk den-
sity of 7.0 g/cm\ a diameter equal to 2 cm, and

Green density, g/cm3
Pressure, MN/m2

5.35
157.5

6.15
315

6.58
472.5

6.75
629.9

6.9
787.4

If it is required to manufacture a gear wheel hav-
ing a green density of 6.8 g/cm3 using a press
with a capacity of 1 MN, calculate the diameter
of the largest gear wheel that can be manufac-
tured. How can you produce a larger gear by
modifying the design?

a height equal to 3 cm. After sintering, its bulk
density increased to 7.05 g/cm3. Calculate its
new dimensions.

9. The sintered density of atomized iron compact
containing 10 percent copper was 7.2 g/cm3.
What is the porosity?

Design Project

Figure 7.14 shows a part that is currently produced by forging and subsequent ma-
chining. Because the part is not subjected to high stresses during its actual service con-
ditions, the producing company is considering the idea of manufacturing it by powder
metallurgy in order to increase the production rate. Redesign this component so that it
can be manufactured by the conventional die-pressing technique.

FIGURE 7.14

A part to be redesigned
for production by
powder metallurgy

Chapter 8

INTRODUCTION

Plastics, which are more correctly called polymers, are products of macromo-
lecular chemistry. In fact, the term polymer is composed of the two Greek
words poly and meres, which mean "many parts." This is, indeed, an accurate
description of the molecule of a polymer, which is made up of a number of iden-
tical smaller molecules that are repeatedly linked together to form a long chain.
As an example, consider the commonly used polymer polyethylene, which is
composed of many ethylene molecules (C2H4) that are joined together, as
shown in Figure 8.1. These repeated molecules are always organic compounds,
and, therefore, carbon usually forms the backbone of the chain. The organic
compound whose molecules are linked together (like ethylene) is referred to as
the monomer.

Now, let us examine why the molecules of a monomer tend to link together.
We know from chemistry that carbon has a valence of 4. Therefore, each car-
bon atom in an ethylene molecule has an unsaturated valence bond. Conse-
quently, if two ethylene molecules attach, each to one side of a third molecule,
the valence bonds on the two carbon atoms of the center molecule will be sat-
isfied (see Figure 8.1). In other words, the molecules of the monomer tend to
attach to one another to satisfy the valence requirement of the carbon atoms.

The molecules of a monomer in a chain are strongly bonded together. Nev-
ertheless, the long chains forming the polymer molecules tend to be more or
less amorphous and are held together by weaker secondary forces that are
known as the van der Waals forces (named after the Dutch physicist). There-

278

8.1 Classification of Polymers

279

FIGURE 8.1

The molecular chain of
polyethylene

H H

H , H

r__

H I H

H i H H

1 I

H H

fore, polymers are generally not as strong as metals or ceramics. It is also ob-
vious that properties of a polymer such as the strength, elasticity, and relax-
ation are dependent mainly upon the shape and size of the long chainlike
molecules, as well as upon the mutual interaction between them.

A common, but not accurate, meaning of the term polymer involves syn-
thetic organic materials that are capable of being molded. Actually, polymers
form the building blocks of animal life; proteins, resins, shellac, and natural
rubber are some examples of natural polymers that have been in use for a long
time. On the other hand, the synthetic, or manufactured, polymers have come
into existence fairly recently. The first synthetic polymer, cellulose nitrate (cel-
luloid), was prepared in 1869. It was followed in 1909 by the phenolics, which
were used as insulating materials in light switches. The evolution of new poly-
mers was accelerated during World War II due to the scarcity of natural materi-
als. Today, there are thousands of polymers that find application in all aspects
of our lives.

LASSIFICATION OF POLYMERS

There are, generally, two methods for classifying polymers. The first method involves
grouping all polymers based on their elevated-temperature characteristics, which actu-
ally dictate the manufacturing method to be used. The second method of classification
groups polymers into chemical families, each of which has the same monomer. As an
example, the ethenic family is based on ethylene as the monomer, and different poly-
mers (members of this family such as polyvinyl alcohol or polystyrene) can be made
by changing substituent groups on the basic monomer, as shown in Figure 8.2. As we
will see later, this enables us to study most polymeric materials by covering just a lim-
ited number of families instead of considering thousands of polymers individually. But
before reviewing the commonly used chemical families of polymers, let us discuss in
depth their elevated-temperature behavior. Based on this behavior, polymers can be
split into two groups: thermoplastics and thermosets.

280

8 Plastics

FIGURE 8.2

Structural formula of some polymers of the ethenic group

H CC

H H

Polyvinyl chloride

— C

-c —

Polyv

nyl

alcohol

H CH,

H H

Polypropylene

Thermoplastics

Thermoplastics generally have linear structures, meaning that their molecules look
like linear chains having little breadth but significant length. This structure, as shown
in Figure 8.3, is analogous to a bowl of spaghetti. Bonds between the various molecu-
lar chains are mainly of the van der Waals type (i.e., secondary forces). Therefore, this
type of polymer softens by heating and can then flow viscously to take a desired shape
because elevated temperatures tend to decrease the intermolecular coherence of the lin-
ear chains. When the solidified polymers are reheated and melted again, they can be
given a different shape. This characteristic enables plastics fabricators to recycle ther-
moplastic scrap, thus increasing the efficiency of raw-material utilization.

Usually, a thermoplastic polymer consists of a mixture of molecular chains having
different lengths. Therefore, each structure has a different melting point, and, conse-
quently, the whole polymer melts, not at a definite temperature, but within a range
whose limits are referred to as the softening point and the flow point. It has been ob-
served that when a thermoplastic is given a shape at a temperature between the soft-

FIGURE 8.3

The molecular chains of
a thermoplastic
polymer

FIGURE 8.4

The molecular chains of
a thermosetting
polymer

8.1 Classification of Polymers 281

ening and the flow points, the intermolecular tension is retained after the thermoplas-
tic cools down. Therefore, if the part is reheated to a temperature above the softening
point, it will return to its original shape because of this intermolecular tension. This
phenomenon, which characterizes most thermoplastic polymers, is known as shaping
memory.

Many thermoplastic polymers are soluble in various solvents. Consequently, any
one of these polymers can be given any desired shape by dissolving it into an appro-
priate solvent and then casting the viscous solution in molds. When the solvent com-
pletely evaporates, it leaves the rigid resin with the desired shape.

Several chemical families of polymeric materials can be categorized as thermo-
plastic. These include the ethenics, the polyamides, the cellulosics, the acetals, and the
polycarbonates. Their characteristics, methods of manufacture, and applications are
discussed in detail later in this chapter.

Thermosets

The molecules of thermosets usually take the form of a three-dimensional network
structure that is mostly cross-linked, as shown in Figure 8.4. When raw (uncured)
thermosetting polymers are heated to elevated temperatures, they are set, cross-
linked, or polymerized. If reheated after this curing operation, thermosets will not
melt again but will char or burn. Therefore, for producing complex shapes of ther-
mosetting polymers, powders (or grains) of the polymers are subjected to heat and
pressure until they are cured as finished products. Such polymers are referred to as
heat-convertible resins.

Some raw thermosets can take the form of liquids at room temperature. When re-
quired, they are converted into solids by curing as a result of heating and/or additives
(hardeners). This characteristic enables fabricators to produce parts by casting mix-
tures of liquid polymers and hardeners into molds. Therefore, these polymers are re-
ferred to as casting resins.

The cured thermosets are insoluble in solvents and do not soften at high tempera-
tures. Thus, products made of thermosets can retain their shape under combined load
and high temperatures, conditions that thermoplastics cannot withstand.

282 8 Plastics

PROPERTIES CHARACTERIZING
PLASTICS AND THEIR EFFECT
ON PRODUCT DESIGN

Properties of plastics differ significantly from those of metals, and they play a very im-
portant role in determining the form of the product. In other words, the form is dictated
not only by the function but also by the properties of the material used and the method
of manufacture, as we will see later. Following is a discussion of the effect of the prop-
erties characterizing plastics on the design of plastic products.

Mechanical Properties

The mechanical properties of polymers are significantly inferior to those of metals.
Strength and rigidity values for plastics are very low compared with the lowest values
of these properties for metals. Therefore, larger sections must be provided for plastic
products if they are to have a similar strength and/or rigidity as metal products. Un-
fortunately, these properties get even worse when plastic parts are heated above mod-
erate temperatures. In addition, some plastics are extremely brittle and notch-sensitive.
Accordingly, any stress raisers like sharp edges or threads must be avoided in such
cases.

A further undesirable characteristic of plastics is that they tend to deform contin-
ually under mechanical load even at room temperature. This phenomenon is acceler-
ated at higher temperatures. Consequently, structural components made of plastics
should be designed based on their creep strength rather than on their yield strength.
This dictates a temperature range in which only a plastic product can be used. It is ob-
vious that such a range is dependent principally upon the kind of polymer employed.

In spite of these limitations, the strength-to-weight ratio as well as the stiffness-to-
weight ratio of plastics can generally meet the requirements for many engineering ap-
plications. In fact, the stiffness-to-weight ratio of reinforced polymers is comparable to
that of metals like steel or aluminum.

Physical Properties

Three main physical properties detrimentally affect the widespread industrial applica-
tion of polymers and are not shared by metals. First, plastics usually have a very high
coefficient of thermal expansion, which is about ten times that of steel. This has to be
taken into consideration when designing products involving a combination of plastics
and metals. If plastics are tightly fastened to metals, severe distortion will occur when-
ever a significant temperature rise takes place. Second, some plastics are inflammable
(i.e., not self-extinguishing) and keep burning even after the removal of the heat
source. Third, some plastics have the ability to absorb large amounts of moisture from
the surrounding atmosphere. This moisture absorption is unfortunately accompanied
by a change in the size of the plastic part. Nylons are a typical example of this kind of
polymer.

8.3 Polymeric Systems

283

►OLYMERIC SYSTEMS

This section surveys the commonly used polymeric materials and discusses their man-
ufacturing properties and applications. Also discussed are the different additives that
are used to impart certain properties to the various polymers.

Commonly Used Polymers

Following are some polymeric materials that are grouped into chemical families ac-
cording to their common monomer.

Ethenic group. The monomer is ethylene. This group includes the following
polymers:

1. Polyethylene.

— c —

The properties of polyethylene depend upon factors like degree of crystallinity,
density, molecular weight, and molecular weight distribution. This thermoplastic
polymer is characterized by its chemical resistance to solvents, acids, and alkalies, as
well as by its toughness and good wear properties. Polyethylenes also have the ad-
vantage of being adaptable to many processing techniques, such as injection mold-
ing, blow molding, pipe extrusion, wire and cable extrusion, and rotational molding.

The applications of polyethylene are dependent upon the properties, which, in
turn, depend upon the density and molecular weight. Low-density polyethylene is
used in manufacturing films, coatings, trash bags, and throwaway products. High-
density polyethylene is used for making injection-molded parts, tubes, sheets, and
tanks that are used for keeping chemicals. The applications of the ultrahigh molec-
ular weight (UHMW) polyethylene include wear plates and guide rails for filling
and packaging equipment.

2. Polypropylene.

H H

H Chi,

284

8 Plastics

Polypropylene is a thermoplastic material. A molecule of this polymer has all
substituent groups (i.e., CH3) on only one of its sides. This promotes crystallinity
and, therefore, leads to strength higher than that of polyethylene. The resistance of
polypropylene to chemicals is also good.

Polypropylene is mainly used for making consumer goods that are subjected to
loads during their service life, such as ropes, bottles, and parts of appliances. This
polymer is also used in tanks and conduits because of its superior resistance to
chemicals.

3. Polybutylene.

H CH3

C C-

H CHo

Polybutylene is a thermoplastic polymer that has high tear, impact, and creep
resistances. It also possesses good wear properties and is not affected by chemicals.
Polybutylene resins are available in many grades, giving a wide range of properties
and, therefore, applications.

The properties of polybutylene have made it an appropriate material for piping
applications. These pipes can be joined together by heat fusion welding or by me-
chanical compression. Some grades of polybutylene are used as high-performance
films for food packaging and industrial sheeting.

4. Polyvinyl chloride.

H H

H CI

Polyvinyl chloride (PVC) is a thermoplastic polymer that can be processed by
a variety of techniques like injection molding, extrusion, blow molding, and com-
pression molding. It is fairly weak and extremely notch-sensitive but has excellent
resistance to chemicals. When plasticized (i.e., additives are used to lubricate the
molecules), it is capable of withstanding large strains.

The applications of rigid PVC include low-cost piping, siding, and related pro-
files, toys, dinnerware, and credit cards. Plasticized PVC is used in upholstery, im-
itation leather for seat covers and rainwear, and as insulating coatings on wires.

8.3 Polymeric Systems

285

5. Polyvinyleidene chloride.

CH2 C -

Polyvinyleidene chloride (PVDC) is nonpermeable to moisture and oxygen. It
also possesses good creep properties. It is a preferred food-packaging material
(e.g., saran wrap). Rigid grades are used for hot piping.

6. Polystyrene.

CH,

This thermoplastic polymer is known as "the cheap plastic." It has poor me-
chanical properties, can tolerate very little deflection, and breaks easily. Because of
its cost, polystyrene is used for cheap toys and throwaway articles. It is also made
in the form of foam (Styrofoam) for sound attenuation and thermal insulation.

7. Polymethyl methacrylate (Plexiglas acrylics).

CH
CH2 C

O OCH3

286

8 Plastics

This polymer has reasonably good toughness, good stiffness, and exceptional
resistance to weather. In addition, it is very clear and has a white-light transmission
equal to that of clear glass. Consequently, this polymer finds application in safety
glazing and in the manufacture of guard and safety glasses. It is also used in mak-
ing automotive and industrial lighting lenses. Some grades are used as coatings and
lacquers on decorative parts.

8. Fluorocarbons like polytetrafluoroethylene (Teflon).

F F

Teflon is characterized by its very low coefficient of friction and by the fact
that even sticky substances cannot adhere to it easily. It is also the most chemi-
cally inert polymer. Nevertheless, it has some disadvantages, including low
strength and poor processability. Because of its low coefficient of friction, Teflon
is commonly used as a dry film lubricant. It is also used as lining for chemical
and food-processing containers and conduits.

Polycarbonate group. These are actually polyesters. They are thermoplastic and have
linear molecular chains. Polycarbonate exhibits good toughness, good creep resistance,
and low moisture absorption. It also has good chemical resistance. It is widely used in
automotive and medical and food packaging because of its cost effectiveness. It is also
considered to be a high-performance polymer and has found application in the form of
solar collectors, helmets, and face shields.

Polyacetal group. Included in this group is formaldehyde, with ending groups.

HOCH,

CH,OH

Formaldehyde is a thermoplastic polymer that can be easily processed by injection
molding and extrusion. It has a tendency to be highly crystalline, and, as a result, this
polymer possesses good mechanical properties. It also has good wear properties and a
good resistance to moisture absorption.

Its applications involve parts that were made of nonferrous metals (like zinc,
brass, or aluminum) by casting or stamping. These applications are exemplified by
shower heads, shower-mixing valves, handles, good-quality toys, and lawn sprinklers.

8.3 Polymeric Systems

287

Cellulosic group. The monomer is cellulose.

CH,OH

H OH

Cellulose itself is not a thermoplastic polymer. It can be produced by the viscous
regeneration process to take the form of a fiber as in rayon, or a thin film, as in cello-
phane. Cellophane applications involve mainly decoration. Nevertheless, cellulose can
be chemically modified to produce the following thermoplastics:

1. Cellulose nitrate. Good dimensional stability and low water absorption are the posi-
tive characteristics of this polymer. The major disadvantage that limits its widespread
use is its inflammability. Cellulose nitrate is used in making table-tennis balls, fash-
ion accessories, and decorative articles. It is also used as a base for lacquer paints.

2. Cellulose acetate. This polymer has good optical clarity, good dimensional stabil-
ity, and resistance to moisture absorption. The uses of cellulose acetate include
transparent sheets and films for graphic art, visual aids, and a base for photographic
films. It is also used in making domestic articles.

3. Cellulose acetate butyrate. This thermoplastic polymer is tough, has good surface
quality and color stability, and can readily be vacuum formed. It finds popular use
in laminating with thin aluminum foil.

4. Cellulose acetate propionate. This thermoplastic polymer has reasonably good
mechanical properties and can be injection molded or vacuum formed. It is used for
blister packages, lighting fixtures, brush handles, and other domestic articles.

Polyamide group. This family includes high-performance melt-processable thermo-
plastics.

NHR

R is a chemical group that differs for different members of this family.

One group of common polyamides is the nylons. These are characterized by their
endurance and retention of their good mechanical properties even at relatively high
temperatures. They also possess good lubricity and resistance to wear. The chief limi-
tation is their tendency to absorb moisture and change size.

288

8 Plastics

These polymers find use in virtually every market (e.g., automotive, electrical,
wire, packaging, and appliances). Typical applications include structural components
up to 10 pounds (4 kg), bushings, gears, cams, and the like.

ABS. The three monomers are acrylonitrile, butadiene, and styrene. Based on this
three-monomer system (similar to an alloy in the case of metals), the properties of this
group vary depending upon the components. Fifteen different types are commercially
used. They possess both good mechanical properties and processability. Applications
of the ABS group include pipes and fittings, appliances and automotive uses, tele-
phones, and components for the electronics industry.

Polyesters. These polymers result from a condensation reaction of an acid and an al-
cohol. The type and nature of the polymer obtained depend upon the acid and alcohol
used. This multitude of polymers are mostly thermoplastic and can be injection molded
and formed into films and fibers. Their uses include bases for coatings and paints,
ropes, fabrics, outdoor applications, construction, appliances, and electrical and elec-
tronic components. Polyester is also used as a matrix resin for fiberglass to yield the
composite fiber-reinforced polymer.

Phenolic group. The monomer is phenol formaldehyde.

As previously mentioned, phenolics are actually the oldest manufactured ther-
mosetting polymers. They are processed by compression molding, where a product
with a highly cross-linked chain structure is finally obtained. Phenolics are character-
ized by their high strength and their ability to tolerate temperatures far higher than
their molding temperature.

Phenolics are recommended for use in hostile environments that cannot be toler-
ated by other polymers. They are used in electrical switchplates, electrical boxes, and
similar applications. Nevertheless, the chief field of application is as bonding agents
for laminates, plywood-grinding wheels, and friction materials for brake lining.

Polyimides. Polyimides are mostly thermosetting and have very complex structures.
They are considered to be one of the most heat-resisting polymers. They do not melt

8.3 Polymeric Systems 289

and flow at elevated temperatures and are, therefore, manufactured by powder metal-
lurgy techniques.

The polyimides are good substitutes for ceramics. Applications include jet-engine
and turbine parts, gears, coil bobbins, cages for ball bearings, bushings and bearings,
and parts that require good electrical and thermal insulation.

Epoxies. Epoxies and epoxy resins are a group of polymers that become highly cross-
linked by reaction with curing agents or hardeners. These polymers have low molecu-
lar weight and got their name from the epoxide group at the ends of the molecular
chains. Epoxy resins are thermosetting and have good temperature resistance. They ad-
here very well to a variety of substrates. Another beneficial characteristic is their sta-
bility of dimensions upon curing.

The common application of epoxy resins is as adhesives. With the addition of
fibers and reinforcements, laminates and fiber-reinforced epoxy resins can be obtained
and are used for structural applications.

Polyurethanes. Polyurethanes involve a wide spectrum of polymers ranging from
soft thermoplastic elastomers to rigid thermosetting foams. While all polyurethanes are
products of a chemical reaction of an isocyanate and an alcohol, different polymers are
apparently obtained by different reacting materials.

Elastomers are used as die springs, forming-die pads, and elastomer-covered rolls.
Some of these elastomers are castable at room temperature and find popular applica-
tion in rubber dies for the forming of sheet metals. Flexible foam has actually replaced
latex rubber in home and auto seating and interior padding. The rigid thermosetting
foam is used as a good insulating material and for structural parts. Other applications
of polyurethanes include coating, varnishes, and the like.

Silicones. In this group of polymers, silicon forms the backbone of the molecular
chain and plays the same role as that of carbon in other polymers.

Silicones can be oils, elastomers, thermoplastics, or thermosets, depending upon
the molecular weight and the functional group. Nevertheless, they are all characterized
by their ability to withstand elevated temperatures and their water-repellent properties.

Silicones in all forms are mainly used for high-temperature applications. These in-
clude binders for high-temperature paints and oven and good-handling tubing gaskets.
Silicone oils are used as high-temperature lubricants, mold release agents, and damp-
ing or dielectric fluids.

Elastomers. These polymeric materials possess a percentage elongation of greater
than 100 percent together with significantly high resilience. This rubberlike behavior
is attributed to the branching of the molecular chains. Elastomers mainly include five

290 8 Plastics

polymers: natural rubber, neoprene, silicone rubber, polyurethane, and fiuoroelas-
tomers. Natural rubber is extracted as thick, milky liquid from a tropical tree. Next,
moisture is removed, additives (coloring, curing agents, and fillers) are blended with it,
and the mixture is then vulcanized. The latter operation involves heating up to a tem-
perature of 300°F (150°C) to start cross-linking and branching reactions.

The application of elastomers includes seals, gaskets, oil rings, and parts that pos-
sess rubberlike behavior such as tires, automotive and aircraft parts, and parts in form-
ing dies.

Additives

Additives are materials that are compounded with polymers in order to impart and/or
enhance certain physical, chemical, manufacturing, or mechanical properties. They are
also sometimes added just for the sake of reducing the cost of products. Commonly
used additives include fillers, plasticizers, lubricants, colorants, antioxidants, and sta-
bilizers.

Fillers. Fillers involve wood flour, talc, calcium carbonate, silica, mica flour, cloth,
and short fibers of glass or asbestos. Fillers have recently gained widespread industrial
use as a result of the continued price increase and short supply of resin stocks. An ex-
pensive or unavailable polymer can sometimes be substituted by another filled poly-
mer, provided that an appropriate filler material is chosen.

The addition of inorganic fillers usually tends to increase the strength because this
kind of additive inhibits the mobility of the polymers' molecular chains. Nevertheless,
if too much filler is added, it may create enclaves or weak spots and cause problems
during processing, especially if injection molding is employed.

Plasticizers. Plasticizers are organic chemicals (high-boiling-temperature solvents)
that are admixed with polymers in order to enhance resilience and flexibility. This is a re-
sult of facilitating the mobility of the molecular chains, thus enabling them to move eas-
ily relative to one another. On the other hand, plasticizers reduce the strength. Therefore,
a polymer that meets requirements without the addition of plasticizers is the one to use.

Lubricants. Lubricants are chemical substances that are added in small quantities to
the polymer to improve processing and flowability. They include fatty acids, fatty al-
cohols, fatty esters, metallic stearates, paraffin wax, and silicones. Lubricants are clas-
sified as external (applied externally to the polymer), internal, or internal-external. The
last group includes most commercially used lubricants.

Colorants. Colorants may be either dyes or pigments. Dyes have smaller molecules
and are transparent when dissolved. Pigment particles are relatively large (over 1 |im)
and are, therefore, either translucent or opaque. Pigments are more widely used than
dyes because dyes tend to extrude from the polymers.

Antioxidants. The use of antioxidants is aimed at enhancing the resistance to oxida-
tion and degradation of polymers, thus extending their useful temperature range and
service life. These substances retard the chemical reactions that are caused by the pres-
ence of oxygen.

8.4 Processing of Plastics 291

Stabilizers. Stabilizers are substances that are added to polymers to prevent degrada-
tion as a result of heat or ultraviolet rays. The mechanism of inhibiting degradation of
polymers differs for different stabilizers. However, ultraviolet stabilizers usually func-
tion by absorbing ultraviolet radiation.

8.4 PROCESSING OF PLASTICS

A variety of processing methods can be employed in manufacturing plastic products.
However, it must be kept in mind that no single processing method can successfully be
employed in shaping all kinds of plastics. Each process has its own set of advantages
and disadvantages that influence product design. Following is a survey of the common
methods for plastic processing.

Casting

Casting is a fairly simple process that requires no external force or pressure. It is usually
performed at room temperature and involves filling the mold cavity with monomers or
partially polymerized syrups and then heating to cure. After amorphous solidification,
the material becomes isotropic, with uniform properties in all directions. Nevertheless,
a high degree of shrinkage is experienced during solidification and must be taken into
consideration when designing the mold. Sheets, rods, and tubes can be manufactured by
casting, although the typical application is in trial jigs and fixtures as well as in insulat-
ing electrical components. Acrylics, epoxies, polyesters, polypropylene, nylon, and
PVC can be processed by casting. The casting method employed is sometimes modified
to suit the kind of polymer to be processed. Whereas nylon is cast in its hot state after
adding a suitable catalyst, PVC film is produced by solution casting. This process in-
volves dissolving the PVC into an appropriate solvent, pouring the solution on a sub-
strate, and allowing the solvent to evaporate in order to finally obtain the required film.

Blow Molding

Blow molding is a fast, efficient method for producing hollow containers of thermo-
plastic polymers. The hollow products manufactured by this method usually have thin
walls and range in shape and size from small, fancy bottles to automobile fuel tanks.
Although there are different versions of the blow molding process, they basically
involve blowing a tubular shape (parison) of heated polymer in a cavity of a split mold.
As can be seen in Figure 8.5, air is injected through a needle into the parison, which
expands in a fairly uniform thickness and finally conforms to the shape of the cavity.

Injection Molding

Injection molding is the most commonly used method for mass production of plastic
articles because of its high production rates and the good control over the dimensions
of the products. The process is used for producing thermoplastic articles, but it can also
be applied to thermosets. The main limitation of injection molding is the required high

292

8 Plastics

FIGURE 8.5

The blow molding
process

Separation
line

View normal to
the separation line

initial capital cost, which is due to the expensive machines and molds employed in the
process.

The process basically involves heating the polymer, which is fed from a hopper in
granular pellet or powdered forms, to a viscous melted state and then forcing it into a
split-mold cavity, where it hardens under pressure. Next, the mold is opened, and the
product is ejected by a special mechanism. Molds are usually made of tool steel and
may have more than a single cavity.

Figure 8.6 shows a modern screw-preplasticator injection unit employed in injection
molding of thermoplastics. As can be seen, the diverter valve allows the viscous polymer
to flow either from the plasticating screw to the pressure cylinder or from the cylinder to
the cooled mold. When thermosets are to be injection molded, a machine with a differ-
ent design has to be used. Also, the molds must be hot so that the polymer can cure.

Once the decision has been made to manufacture a plastic product by injection
molding, the product designer should make a design that facilitates and favors this
process. Following are some design considerations and guidelines.

Make the thickness of a product uniform and as small as possible. Injection mold-
ing of thermoplastics produces net-shaped parts by going from a liquid state to a solid
state. (These net-shaped parts are used as manufactured; they do not require further
processing or machining.) This requires time to allow the heat to dissipate so that the

FIGURE 8.6

The injection molding
process

Heaters

Split
mold

njection
plunger

8.4 Processing of Plastics

293

polymer melt can solidify. The thicker the walls of a product, the longer the product
cycle, and the higher its cost. Consequently, a designer has to keep the thickness of a
product to a minimum without jeopardizing the strength and stiffness considerations.
Also, thickness must always be kept uniform; if change in thickness is unavoidable, it
should be made gradually. It is better to use ribs rather than increase the wall thickness
of a product. Figure 8.7 shows examples of poor design and how they can be modified
(by slight changes in constructional details) so that sound parts are produced.

Provide generous fillet radii. Plastics are generally notch-sensitive. The designer
should, therefore, avoid sharp corners for fillets and provide generous radii instead.
The ratio of the fillet radius to the thickness should be at least 1 .4.

Ensure that holes will not require complex tooling. Holes are produced by using core
pins. It is, therefore, clear that through holes are easier to make than blind holes. Also,
when blind holes are normal to the flow, they require retractable core pins or split
tools, thus increasing the production cost.

FIGURE 8.7

Examples of poor and
good designs of walls
of plastic products

V&77A WZZ&

Yes

Better

Rib

Improved design

V////////////A

Poor

1.5/

Good

Better

294

8 Plastics

FIGURE 8.8

Examples of poor and
good designs of bosses
in injection-molded
parts

^y\

77777

Poor design

Good design

Through holes are better than blind holes

Provide appropriate draft. As is the case with forging, it is important to provide a
draft of 1 ° so that the product can be injected from the mold.

Avoid heavy sections when designing bosses. Heavy sections around bosses lead to
wrappage and dimensional control problems. Figure 8.8 shows poor and good designs
of bosses.

Compression Molding

Compression molding is used mainly for processing thermosetting polymers. The
process involves enclosing a premeasured charge of polymer within a closed mold and
then subjecting that charge to combined heat and pressure until it takes the shape of the
mold cavity and cures. Figure 8.9 shows a part being produced by this process.

Although the cycle time for compression molding is very long when compared
with that for injection molding, the process has several advantages. These include low
capital cost (because the tooling and the equipment used are simpler and cheaper) and
the elimination of the need for sprues or runners, thus reducing the material waste. There

8.4 Processing of Plastics

295

FIGURE 8.9

The compression moldin
process

Flash

Part

are, however, limitations upon the shape and size of the products manufactured by this
method. It is generally difficult to produce complex shapes or large parts as a result of
the poor flowability and long curing times of the thermosetting polymers.

Transfer Molding

Transfer molding is a modified version of the compression molding process, and it is
aimed at increasing the productivity by accelerating the production rate. As can be
seen in Figure 8.10, the process involves placing the charge in an open, separate "pot,"
where the thermosetting polymer is heated and forced through sprues and runners to
fill several closed cavities. The surfaces of the sprues, runners, and cavities are kept at
a temperature of 280 to 300°F (140 to 200°C) to promote curing of the polymer. Next,
the entire shot (i.e., sprues, runners, product, and the excess polymer in the pot) is
ejected.

Rotational Molding

Rotational molding is a process by which hollow objects can be manufactured from
thermoplastics and sometimes thermosets. It is based upon placing a charge of solid or
liquid polymer in a mold. The mold is heated while being rotated simultaneously
around two perpendicular axes. As a result, the centrifugal force pushes the polymer
against the walls of the mold, thus forming a homogeneous layer of uniform thickness

FIGURE 8.10

The transfer molding
process

Plunger

Part

Runner

296

8 Plastics

FIGURE 8.11

The extrusion process

Changeable die

Extruded section

Heater

that conforms to the shape of the mold, which is then cooled before the product is
ejected. The process, which has a relatively long cycle time, has the advantage of of-
fering almost unlimited product design freedom. Complex parts can be molded by em-
ploying low-cost machinery and tooling.

Extrusion

In extrusion, a thermoplastic polymer in powdered or granular form is fed from a hop-
per into a heated barrel, where the polymer melts and is then extruded out of a die. Fig-
ure 8.11 shows that plastics extrusion is a continuous process capable of forming an
endless product that has to be cooled by spraying water and then cut to the desired
lengths. The process is employed to produce a wide variety of structural shapes, such
as profiles, channels, sheets, pipes, bars, angles, films, and fibers. Extrusions like bars,
sheets, and pipes can also be further processed by other plastic manufacturing methods
until the desired final product is obtained.

A modification of conventional extrusion is a process known as coextrusion. It in-
volves extruding two or more different polymers simultaneously in such a manner that
one polymer flows over and adheres to the other polymer. This process is used in in-
dustry to obtain combinations of polymers, each contributing some desired property.
Examples of coextrusion include refrigerator liners, foamed-core solid-sheath tele-
phone wires, and profiles involving both dense material and foam, which are usually
used as gasketing in automotive and appliance applications.

Thermoforming

Thermoforming involves a variety of processes that are employed to manufacture cup-
like products from thermoplastic sheets by a sequence of heating, forming, cooling,
and trimming. First, the sheet is clamped all around and heated to the appropriate tem-
perature by electric heaters located above it. Next, the sheet is stretched under the ac-
tion of pressure, vacuum, or male tooling and is forced to take the shape of a mold.
The polymer is then cooled to retain the shape. This is followed by removing the part
from the mold and trimming the web surrounding it. Figure 8.12a through d illustrates
the different thermoforming processes.

Although thermoforming was originally developed for the low-volume production
of containers, the process can be automated and made suitable for high-volume appli-
cations. In this case, molds are usually made of aluminum because of its high thermal

8.4 Processing of Plastics

297

FIGURE 8.12

Different
thermoforming
processes: (a) straight
vacuum forming;

(b) drape forming;

Heater

Original
sheet

Heater

-■•»fi"»-» in »»-'

v ^---CLOngina

sheet
Mold

(Upper half
of mold)

Lower half
of mold)

atmosphere ~|

Vent for

relieving

entrapped air

Heated
plastic
sheet

Vacuum

[1) First stage

Vacuum

¥fm

MmtM

Atmosphere

(2) Second stage
(d)

conductivity. For low-volume or trial production, molds are made of wood or even
plaster of paris.

Examples of the parts produced by thermoforming include containers, panels,
housings, machine guards, and the like. The only limitation on the shape of the prod-
uct is that it should not contain holes. If holes are absolutely required, they should be
made by machining at a later stage.

Calendering

Calendering is the process employed in manufacturing thermoplastic sheets and films.
This process is similar to rolling with a four-high rolling mill, except that the rolls that
squeeze the polymer are heated. The thermoplastic sheet is fed and metered in the first
and second roll gaps, whereas the third roll gap is devoted to gaging and finishing.

298 8 Plastics

FIGURE 8.13

The calendering
process

Most of the calendering products are flexible or rubberlike sheets and films, although
the process is sometimes applied to ABS and polyethylene. Figure 8.13 illustrates the
calendering process.

Machining of Plastics

In some cases, thermoplastic and thermosetting polymers are subjected to machining
operations like sawing, drilling, or turning. Some configurations and small lot sizes can
be more economically achieved by machining than by any other plastic-molding
method. Nevertheless, there are several problems associated with the machining of plas-
tics. For instance, each type of plastic has its own unique machining characteristics, and
they are very different from those of the conventional metallic materials. A further prob-
lem is the excessive tool wear experienced when machining plastics, which results in the
interruption of production as well as additional tooling cost. Although much research is
needed to provide solutions for these problems, there are some general guidelines:

1. Reduce friction at the tool-workpiece interface by using tools with honed or pol-
ished surfaces.

2. Select tool geometry so as to generate continuous-type chips. Recent research has
revealed that there exists a critical rake angle (see Chapter 9) that depends upon
the polymer, depth of cut, and cutting speed.

3. Use twist drills that have wide, polished flutes, low helix angles, and tool-point
angles of about 70° and 120°.

Recently, lasers have been employed in cutting plastics. Because a laser acts as a ma-
terials eliminator, its logical application is cutting and hole drilling. High-pressure water
jets also currently find some application in the cutting of polymers and composites.

Welding of Plastics

There are several ways for assembling plastic components. The commonly used meth-
ods include mechanical fastening, adhesive bonding, thermal bonding, and ultrasonic
welding. Only thermal bonding and ultrasonic welding are discussed next because the
first two operations are similar to those used with metals.

8.4 Processing of Plastics

299

FIGURE 8.14

Steps involved in hot-
plate joining

Fixtures

Hot
plate

Force

Thermal bonding of plastics. Thermal bonding, which is also known as fusion bond-
ing, involves the melting of the weld spots in the two plastic parts to be joined and then
pressing them together to form a strong joint. Figure 8.14 illustrates the steps involved
in the widely used thermal bonding method known as hot-plate joining. As can be seen
in the figure, a hot plate is inserted between the edges to be mated in order to melt the
plastic parts; melting stops when the plate comes in contact with the holding fixture.
Next, the plate is withdrawn, and the parts are pressed together and left to cool to yield
a strong joint. The cycle time usually ranges from 15 to 20 seconds, depending upon
the relationship between the melt time and the temperature (of the hot plate) for the
type of plastic to be bonded. Also, this process is applied only to thermoplastics.

Figure 8.15 illustrates different types of joint design. The one to select is depen-
dent upon both the desired strength and the appearance of the joint. The product de-
signer must keep in mind that a small amount of material is displaced from each side
to form the weld bead. This must be taken into account when dimensional tolerance is
critical, such as when fusion-bonded parts are to be assembled together.

Another thermal bonding process, which is equivalent to riveting in the case of
metals, is referred to as the rmo staking. As can be seen in Figure 8.16, the process

FIGURE 8.15

Different joint designs
for fusion bonding

Straight
butt joint

Flanged
butt joint

Bead enclosed

Bead covered

Recessed weld

300

8 Plastics

FIGURE 8.16

The thermostaking
process

Hot air

involves the softening of a plastic stud by a stream of hot air and then forming the
softened stud and holding it while it cools down. Thermal bonding processes find
widespread application in the automotive, appliance, battery, and medical industries.

Ultrasonic welding of plastics. Ultrasonic welding is gaining popularity in industry
because of its low cycle time of about 0.5 second and the strong, tight joints that are
easily obtainable. The process is used for thermoplastics and involves conversion of
high-frequency electrical energy to high-frequency mechanical vibrations that are, in
turn, employed to generate highly localized frictional heating at the interface of the
mating parts. This frictional heat melts the thermoplastic polymer, allowing the two
surfaces to be joined together.

The product designer must bear in mind that not all thermoplastics render them-
selves suitable for ultrasonic welding. Whereas amorphous thermoplastics are good
candidates, crystalline polymers are not suitable for this process because they tend to
attenuate the vibrations. Hydroscopic plastics (humidity-absorbing polymers, such as
nylons) can also create problems and must, therefore, be dried before they are ultra-
sonically welded. In addition, the presence of external release agents or lubricants re-
duces the coefficient of friction, thus making ultrasonic welding more difficult.

The equipment used involves a power supply, a transducer, and a horn. The power
supply converts the conventional 115-V, 60-Hz (or 220- V, 50-Hz) current into a high-
frequency current (20,000 Hz). The transducer is usually a piezoelectric device that
converts the electrical energy into high-frequency, axial-mechanical vibrations. The
horn is the part of the system that is responsible for amplifying and transmitting the
mechanical vibrations to the plastic workpiece. Horns may be made of aluminum,
alloy steel, or titanium. The latter material possesses superior mechanical properties
and is, therefore, used with heavy-duty systems. The horns amplify the mechanical vi-
bration via a continuous decrease in the cross-sectional area and may take different
forms to achieve that goal, as shown in Figure 8.17.

The task of joint design for ultrasonic welding is critical because it affects the de-
sign of the molded parts to be welded. Fortunately, there are a variety of joint designs,
and each has its specific features, advantages, and limitations. The type of joint to be
used should obviously depend upon the kind of plastic, the part geometry, the strength
required, and the desired cosmetic appearance. Following is a discussion of the com-
monly used joint designs, which are illustrated in Figure 8.18.

8.4 Processing of Plastics

301

FIGURE 8.17

Different horn shapes
employed in ultrasonic
welding of plastics

\ r

Catenoidal horn

Step horn

Exponential horn

FIGURE 8.18

Different joint designs
for ultrasonic welding:
(a) butt joint; (b) step
joint; (c) tongue-and-
groove joint; (d)
interference joint;
(e) scarf joint

(a)

(b)

Parts to be
joined

Fixture

(d)

(e)

1. Butt joint with energy director. The butt joint (see Figure 8.18a) is the most
commonly used joint design in ultrasonic welding. As can be seen in the figure,
one of the mating parts has a triangular-shaped projection. This projection is
known as an energy director because it helps to limit the initial contact to a very
small area, thus increasing the intensity of energy at that spot. This causes the
projection to melt and flow and cover the whole area of the joint. This type of
joint is considered to be the easiest to produce because it is not difficult to mold
into a part.

2. Step joint with energy director. The step joint (see Figure 8.18b) is stronger than
the butt joint and is recommended when cosmetic appearance is desired.

302

8 Plastics

FIGURE 8.19

Ultrasonic installation
of metal insert into
plastic part

Metal insert

(diameter bigger

than the hole)

Plastic part

Fixture

3. Tongue-and-groove joint with energy director. The tongue-and-groove joint (see
Figure 8.18c) promotes the self-locating of parts and prevents flash. It is stronger
than both of the previously mentioned methods.

4. Interference joint. The interference joint (see Figure 8.18d) is a high-strength joint
and is usually recommended for square corners or rectangular-shaped parts.

FIGURE 8.20

Ultrasonic staking

Staking
tool

Flared stake
diameter less than ^ in. (1.6 mm)

Spherical stake
diameter less than -^ in. (1.6 mm)

®m

Hollow stake
diameter more than ^- in. (4 mm)

N/S/VV

Knurled stake

(used for high-volume production

and/or where appearance and

strength are not critical)

Flush stake

(recommended when the thickness of

the sheet allows a chamber or a counterbase)

8.5 Fiber-Reinforced Polymeric Composites 303

5. Scarf joint. The scarf joint (see Figure 8.18e) is another high-strength joint and is
recommended for components with circular or oval shapes.

In addition to welding, ultrasonics are employed in inserting metallic parts into
thermoplastic components. Figure 8.19 illustrates an arrangement for the ultrasonic in-
stallation of a metal insert into a plastic part.

Another useful application of these systems is ultrasonic staking, which is equiv-
alent to riveting or heading. Figure 8.20 indicates the different types of stakes, as well
as their recommended applications. Notice that these stakes can be flared, spherical,
hollow, knurled, or flush.

J. 5 FIBER-REINFOR
COMPOSITES

In this present age of new materials, at the forefront of advancing developments are
materials based on the combination of organic polymer resins and high-strength, high-
stiffness synthetic fibers. This section addresses the materials, processing, and design
methodology of fiber-reinforced polymeric composites.

Historical Background

Although the merits of fiber-reinforced materials have been known for centuries,
(straw-reinforced clay was reportedly used as a building material by the Egyptians in
600 B.C.), it is only in the past 40 years that fiber-reinforced polymers have become im-
portant engineering materials. New synthetic high-strength, high-modulus fibers and
new resins and matrix materials have elevated fiber-reinforced composites into the ma-
terial of choice for innovative lightweight, high-strength engineered products. These de-
velopments along with established engineering design criteria and special processing
technology have advanced fiber-reinforced composites close to the realm of a commod-
ity material of construction. In the areas of automobile bodies, recreational boat hulls,
and bathroom fixtures (bathtubs and shower stalls), fiberglass-reinforced organic poly-
mer resins have indeed become the material of choice. In more advanced applications,
the first completely fiber-reinforced polymeric resin composite aircraft came into exis-
tence in the 1980s. For the 1990s, some important nonaerospace applications are emerg-
ing, such as sports equipment (sailboat spars) and, more recently, wind turbine blades.
The utilization of composite materials in functional engineering applications con-
tinues to grow. It is, therefore, important for engineering students to know about and
understand these materials so that new uses may be developed and propagated. Con-
sequently, a brief review of organic polymer engineering composites is presented next.
A general description of these materials, their unique properties, processing tech-
niques, and engineering design features will put into perspective present and future
uses of fiber-reinforced polymer (FRP) engineering materials.

* Section 8.5 was written by Dr. Armand F. Lewis, Lecturer at the University of Massachusetts Dartmouth.

304

8 Plastics

Nature of Composites

A composite may be defined as a material made up of several identifiable phases, com-
bined in an ordered fashion to provide specific properties different from or superior to
those of the individual materials. Many types of composites exist, including laminated
materials, filamentary-wound or -layered and particulate-filled compositions, and mul-
tiphase alloys and ceramics. Most naturally occurring structural materials are compos-
ites (wood, stone, bone, and tendon).

Overall, composite materials can be classified according to Table 8.1. We will
focus on fiber/resin composite materials composed of higher-strength, higher-modulus
fibers embedded in an organic polymer/resin matrix. Table 8.2 lists some of the com-
mon resin and fiber materials employed. These composite materials are generally re-
ferred to as fiber-reinforced polymers (FRP). Currently, polyester and epoxy resins
are the most common commercially used matrix resin polymers, while glass fibers are
the most widely used reinforcing fiber. Resin matrix composites containing high-
strength, high-elasticity-modulus carbon (graphite), polyaramid (Kevlar, a DuPont
trade name), and boron fibers are also in use for specialty (advanced) composite mate-
rial applications.

The integral combination of high-strength, high-elasticity-modulus fibers and rel-
atively low-strength, low-rigidity polymer matrices forms some unique engineering
materials. FRP composites possess the material processing and fabrication properties
of polymeric materials yet, due to their fiber reinforcement, can be designed to possess
directional stiffness and strength properties comparable to those of metals. These me-
chanical properties can be achieved at a very light weight. This feature can be illus-
trated by comparing the specific strength (tensile strength/density) to the specific
elastic modulus (tensile elasticity-modulus/density) of various fiber-reinforced com-
posite materials with plastics and metals. Figure 8.21 compares the specific strengths
and specific elastic moduli of these materials. Notice that commodity elastomers, plas-

TABLE 8.1

Classifications of
composite materials

Classification

Typical Example(s)

Fiber/resin composites

Glass fabric/mat reinforced polyester

Continuous

resin molded into sport boat hulls

Discontinuous

Heterophase polymer mixtures

Aluminum and/or graphite powder

Random particulate filled

blended into nylon plastic to form a

Flake or shaped particles

machine gear

Interstitial polymeric materials

"Marbleized" decorative plastic for

Interpenetrating polymer networks

wall panels

Skeletal composites

Laminar and linear composites

High-pressure laminates used in

Material hybrids

kitchen countertops and

Polymer-polymer

polyurethane rubber-impregnated

polyaramid rope/cable

8.5 Fiber-Reinforced Polymeric Composites

305

TABLE 8.2

Some materials used in
organic polymer
engineering composites

/ Matrix Resin

Maximum Service Temperature \

Epoxy

Up to 121°C (250°F)

Polyester

Up to 62°C (non HT)

Vinyl ester

Up to 145°C (HT type)

Phenolic

Up to 149°C (300°F)

Polyimide

Up to 260°C (500°F)

Thermoplastics

Nylon

Up to 80°C (175°F)

Polyphenylene sulfide

Up to 149°C (300°F)

Polyetheretherktone (PEEK)

Up to 200°C (392°F)

Fiber (Continuous Yarn/Filament,

Woven Fabric,

Nonwoven Mat, Chopped Fiber)

Glass (especially E and S glass)

Polyaramid organic fiber (Kevlar)®

(Dupont trademark)

Carbon

Boron

Form for Processing

Liquid casting resin

B-stage resin mixture

Preimpregnated (prepreg) B-stage resin/fiber/fabr

ic combination

tics, and metals occupy only a very small portion of this structural materials map.
Fibers and fiber-reinforced resin composites occupy the outer regions. Fiber-reinforced
composites can have specific strengths and moduli up to six times those of common
structural materials. For a given weight, fiber-reinforced composites far outperform
other engineering materials in their strength and stiffness. These specific strengths and
moduli approach the mechanical properties of theoretically perfect, ordered polymer
crystals. This property makes composite materials unique among engineering struc-
tural materials and opens new horizons for novel engineering designs. For example,
composite materials are widely used in aircraft and aerospace applications: The FRP
property of high specific strength with high elasticity modulus made possible the de-
sign, construction, and functional deployment of the U.S. Air Force all-carbon fiber-
reinforced epoxy resin composite Stealth reconnaissance aircraft.

The observed high strength and stiffness-to-weight ratio of fiber-reinforced com-
posites can be easily explained. Various material properties of composites can be esti-
mated by a rule of mixtures approach. Micromechanic properties such as modulus
(stiffness), strength, Poisson's ratio, and thermal expansion of fiber-reinforced polymer
composites can be estimated by the following equation:

Mc = VfMt + VmMn

(8.1)

306

8 Plastics

FIGURE 8.21

Specific strengths and
specific elastic moduli
of materials

1 '

Key

' '

UT77\

Composite (unidirectional)

Graphite

Metals

Glass

Plastics

Boron

Elastomers

Kevlar

High Modulus

Epoxy
HTGr

High Strength —

HMGr

—

K/Ep

B _

~ xmn

>x (HP

■*— B/Ep

. Gl/Ep

Gr/Ep

-<UD

Chopped \^

— ^-^y^^ffiPtx

^^Gr/Ep

—

(py r^l^vr

©W | \^

i_ _L

200 400 600

Specific modulus, inches x 10-6

800

where: M is the particular material property

V is the volume fraction of the fiber (f ) or matrix (m)
Mc is the material property of the composite "mixture"

The individual material component properties, therefore, contribute by a volume frac-
tion ratio to the properties of the combined composite materials. For this rule of mix-
tures equation to apply, several basic assumptions and limitations are involved:

1. The fiber/polymer matrix composites as well as the polymer matrix are assumed
to be linearly elastic and homogeneous.

2. There are no voids in the composite, and there is good adhesion between the rein-
forcing fibers and the polymer matrix.

3. The proximity of the fiber and polymer does not alter the properties of the indi-
vidual components.

4. The rule of mixtures has some directional limitations as many FRPs are not
isotropic materials.

8.5 Fiber-Reinforced Polymeric Composites 307

For example, if we are dealing with a continuous fiber-reinforced polymer resin
composite, the modulus and strength properties of the composite will be very different
in the direction longitudinal to the fiber length compared to the properties across or
perpendicular to the fibers. For strength and modulus, Equation 8.1 is most appropri-
ate for composites being tested in the longitudinal (fiber) direction. The mechanical
contribution of the fibers are directly in line with the direction of pull. The fibers are
strong and stiff in this longitudinal direction, and the polymer matrix is relatively weak
and much less rigid. Note that the strength and stiffness of materials in fiber form
are always much higher than bulk materials (e.g., bar, rod, plate) because the fiber
form of a material has a more atomically ordered internal structure. Fibers have an
internal crystalline structure that favorably alters the stiffness and fracture behavior
of this form of material. The presence of fibers makes composites stiffer and stronger
in the longitudinal (fiber) direction than the polymer matrix by itself. The term fiber-
reinforced polymer is thus appropriate. Property directionality effects are very impor-
tant to consider in the use of fiber-reinforced composites in engineering designs.

Fiber Reinforcement

Generally, reinforcement in FRPs can be either fibers, whiskers, or particles. In composite
materials of the most commercial interest, fibers are the most important and have the most
influence on composite properties. Table 8.3 presents a comparison of the most common re-
inforcement fibers used in preparing organic polymer engineering composites. Nylon fiber
is included here as areference fiber. All the materials listed in Table 8.3 are textile fibers and
can, for the most part, be processed into manufactured products in the same manner as tex-
tile fibers (e.g., continuous yarn, wound filaments, woven and knitted fabrics, nonwoven
mats). The high-strength and high-stiffness properties of the glass (S-2), carbon, and pol-
yaramid fibers are evident. These reinforcing fibers, when used in composite material fab-
rication, can take several forms, such as <0. 1 inch (3^4- mm) fiber "whiskers," 0. 1 -0.3 inch
(3-10 mm) chopped fibers, 0.1-2.0 inch (3-50 mm) (nonwoven) matted fiber sheets,
woven fabric (continuous) fiber with plain weave, and unidirectional/longitudinal (contin-
uous) fiber ribbons. These fiber reinforcement forms are illustrated in Figure 8.22. When
using fiber reinforcement in polymer composites, the surface of the fibers or yarns is pre-

TABLE 8.3

Comparison properties of various fibers

S-2 Glass

Carbon T-300

Aramid-49

Nylon 6/6

Tensile Strength,

665,700

390,100

400,000

143,000

lb/in.2 (MPa)

(4,590)

(2,690)

(2,758)

Modulus of Elasticity,

13,000,000

33,000,000

18,000,000

800,000

lb/in.2 (MPa)

(87,000)

(229,000)

(124,110)

(5,516)

Elongation, %

5.4

1.2

2.5

18.3

Density,

0.090

0.062

0.052

0.041

lb/in.3 (g/cm3)

(2.49)

(1.73)

(1.44)

(1.14)

308

8 Plastics

FIGURE 8.22

Comparison of fiber
reinforcement forms

\ s N ' ' " S-- \ ^ X '

' '

~ ' .", - x ' \ ~~ ' ', -

N ~"

/
1

-. 1 / N ' ^ - __ - 1 /

\ /

, V / - v - ^ X

~ •

Whiskers
<3-4 mm

Chopped fibers
3-1 0 mm

Fiber (non-woven) mat
>3-50 mm

iff

TTUTTUTT

Woven fabric
continuous

Parallel aligned yarns
continuous

8.5 Fiber-Reinforced Polymeric Composites 309

treated with a chemical coupling agent to enhance wetting and adhesion of the matrix resin
to the fibers. Here, the chemical coupling agents are made specific to the chemical nature of
the matrix resin being used. It is important that the fiber supplier be consulted for the proper
type of fiber/resin coupling agent when fiber reinforcement materials are purchased.

As Table 8.3 shows, the most commonly used reinforcing fiber material is glass.
In particular, S-2 glass is used in most high-performance applications. There exists an
extensive applications, manufacturing, and processing history involving the use of glass
fiber in polymer composite applications. Various forms of carbon fiber are also used
for high-performance applications. The processing of carbon-fiber-reinforced polymer
composites follows similar procedures as glass fibers. However, in the continuous-yarn
processing of carbon fibers, precautions must be taken to protect electrical processing
equipment from damage. Airborne, electrically conducting graphite dust is generated
when the carbon fibers or yarns are processed through guide rings and rollers. This can
occur before the fibers are wetted by the matrix resin during material fabrication. The
dust can ruin electrical equipment if it is allowed to penetrate an instrument's enclo-
sure. Sometimes, explosion proof electrical equipment is used when processing carbon
fibers. Another approach is to fit the electrical instrument housing with a positive pres-
sure differential of clean air (or nitrogen gas).

Matrix Resins

Classification of polymer matrices. Many types of polymers and resins can be rein-
forced by fibers to create FRP composite materials. Polymer matrices can be classified
into two basic categories: thermoplastic and thermosetting.

1. Thermoplastic. Many of the polymers previously discussed can be reinforced with
fibers to form composites. The most common types are chopped-fiber-reinforced
thermoplastics. These materials can be processed in the same way as nonfiber-
reinforced plastics. Generally, chopped fibers are blended and mixed into a molten
mass of the engineering thermoplastic (e.g., nylon, polycarbonate, acetal) in a melt-
extruder type of plastics-compounding machine. The fiber containing plastic is ex-
truded into a thin rod and cut into molding powder or pellets. This thermoplastic
molding powder is then used for injection molding or extrusion of engineered parts
similar to the unreinforced plastics discussed in the preceding sections.

Continuous fibers such as glass, carbon, or polyaramid have also been prepared
with thermoplastic resin matrices. The concept here is to first coat thermoplastic
resins onto continuous-fiber yarn by a hot melt or a polymer solution-solvent-dip
process. These thermoplastic polymer-coated yarns can then be fabricated into
shaped structures by a (hot press) matched-die compression molding technique or
other techniques for affecting molten-polymer controlled consolidation. At this
time, discontinuous chopped-fiber thermoplastic composites are much more widely
used than continuous fiber-reinforced composites. The main advantages of thermo-
plastic matrix fiber composites is that they can be processed, for the most part, in
conventional thermoplastic polymer fabrication equipment. Furthermore, any scrap
or off-quality material can be recycled back into the injection molding or extruding
machine. However, care must be taken during this thermoplastic processing not to

310 8 Plastics

overly "work" these materials in the molten state. Excessive processing in the
molten state severely shortens the overall reinforcing fiber length, which can di-
minish the reinforcement effect of the fiber in the polymer matrix.

2. Thermosetting. Reinforced composites are traditionally associated with thermoset-
ting polymers such as the unsaturated polyester and epoxy resins. In their cured
state, thermosetting resins are composed of long polymer chains that are joined to-
gether through cross-bridges that link together all the molecules in the resin mass.
The final, hardened, tough, and glassy state of the cured resin is the terminal con-
dition of the polymer resin matrix. In this state, the resin serves the all-important
role of structurally consolidating, supporting, and cohesively tying together the
fiber reinforcement in the composite. However, during initial processing, it is im-
portant that thermosetting resins undergo a gradual liquid-to-solid conversion. It is
this feature that renders thermosetting resins of the unsaturated polyester and epoxy
type most readily adaptable to fiber-reinforced composite component fabrication.

Sequence of FRP fabrication with respect to the resin system involved. At first, the
resin is in a liquid state as it is received from the supplier. It may be more or less fluid
depending on its viscosity (from a flowable waterlike consistency to a high-viscosity
syrup). At this stage, rheological thickeners to increase resin viscosity or reactive dilu-
ents to decrease resin viscosity may be added to the resin formulation. Frequently, the
curative part of the resin system is much more fluid than the resin part. Here, the vis-
cosity of the final mixed resin and curative system is low enough to accommodate
proper flow in processing. Sometimes, the fluidity of the resin may be lowered by in-
creasing the temperature of the resin upon its application to the fiber. In all, it is im-
portant that the viscosity of the liquid resin be adjusted so that it has the proper fluidity
to wet, impregnate, and saturate the reinforcing fiber yarns, fabric, or mat.

The next consideration is the need to chemically catalyze the resin so that it prop-
erly cross-links and cures the resin under the prescribed conditions. It is also necessary
to have the catalyzed resin react very slowly at ambient temperature so that it remains
fluid while it is in the process of wetting the reinforcing fibers. This resin-system fluid
time is referred to as the pot life or open time of the resin. This fluid-time feature is
controlled by the nature of the catalyst, the ambient temperature, and the bulk volume
of resin in the resin container. Note that a bulk of catalyzed resin is a resin undergoing
a heat-generating exothermic reaction. If the bulk volume of the resin is too large, heat
cannot be easily dissipated. The reaction in the fiber/resin dip tank will automatically
accelerate, the resin will cure, or, worse, the heat of the reaction may cause a serious
fire as well as noxious fumes. Most often, however, the processing equipment will con-
tain dual-component pumps and a mixing head that will continuously meter and mix
the proper components of the resin system (resin: part A; curative: part B) at the ap-
propriate moment and position for wetting the fibers.

Once the resin part and the curative part of the resin system have been mixed, the
liquid-to-solid cure reaction of the resin begins. The curing resin system will undergo
several stages: liquid/fluid, gel stage, rubbery stage, and tough/glassy solid. Depending
upon the processing temperature, the liquid-to-gel-to-rubber transition may occur from
hours (for room temperature cures), to minutes, to seconds. The gel point in a ther-

8.5 Fiber-Reinforced Polymeric Composites

311

mosetting resin system is the point in the cure-time sequence when the resin undergoes
a sharp rise in viscosity and ceases to be a fluid. Theoretically, the gel point is defined
as the time in the cure when each polymer molecule in the system is tied together by
at least one cross-link. Therefore, at the gel point, the polymer molecules in the resin
system have combined and have reached an infinite molecular weight. After the gel
point, the number of cross-links in the polymer system continues to increase, the cross-
link network gets tighter and tighter, and the resin becomes a solid. It is at the gel stage
that the influence of cross-linking takes hold. The rubbery stage is intermediate in
cross-linking. In the solid glassy state of the resin, the ultimate number of cross-links
in the resin system exists. Figure 8.23 illustrates the nature of the polymer resin and
curative molecules during the curing sequence. Note that it is only in the solid-state
stage that the fabricated composite part retains its shape and may be moved for addi-
tional processing or given a postcure if necessary. Let us now examine the specific
resin chemistry of the unsaturated polyester and epoxy resin systems.

Chemistry of the unsaturated polyester resin system. Unsaturated organic polymers
are polymer systems containing double bonds, or C = C. Double bonds can react with
each other by an addition reaction that can be initiated by a free-radical catalyst. With
the help of free-radical catalysts, unsaturated organic compounds can react with each
other to form high-molecular-weight polymers:

l-T

C =

= C

R'_

unsaturated
monomer

catalyst

bond opening

r i

3D O I

H
p

polymer

In unsaturated polyester resins, the resin part of the mixture is represented by high-
molecular-weight polymer molecules having unsaturated groups in their chain. These
unsaturated polyesters are readily soluble in the unsaturated organic liquid compound
called styrene. Styrene (known here as a monomer) can easily react with itself (using a
free-radical catalyst initiator) to form a styrene polymer, or polystyrene. Because the
monomeric styrene can readily react with unsaturated groups, when liquid styrene is
mixed with unsaturated polyester resin, it serves the dual role of a reactive diluent and
cross-linking agent. If a free-radical catalyst is added to a solution mixture of unsatu-
rated polyester resin and styrene, the styrene simultaneously reacts with both the unsat-
uration in the backbone of the polyester chain and with itself. With free-radical catalysis,
the polymerization reaction involving the growing polystyrene chains that react with the
one polyester chain can also react with itself. When this reaction, in turn, connects with
another polyester chain, a cross-link is formed between the two chains. In the molecu-
lar mixture mass of styrene, growing polystyrene chains, and unsaturated polyester mol-
ecules, an array of cross-links are formed between the multitude of polyester molecules
(see Figure 8.24). As the polymer system reacts, from its initial mixing of the catalyst,
the resin system will change from a liquid, to a gel-rubber when cross-linking starts to

312

8 Plastics

FIGURE 8.23

Nature of molecules at
various stages of
thermosetting resin
cure

BACKBONE Polymer (Pre-Polymer)

CROSSLINICING Chemical/Agent

Liquid - All molecules
are independent,
can flow past each
other

Gel - At least one
crosslink attachment
to each backbone
ploymer molecule.

Rubber - More
crosslinks, backbone
still flexible

• Glass - High
crosslink density,
tight network
structure

8.5 Fiber-Reinforced Polymeric Composites

313

FIGURE 8.24

Chemical structure of
unsaturated
polyester/styrene resin

H H

• H

<{> H

Unsaturated

polyester resin

(UP)

Styrene
(Monomer)

H H H H H
/™ — c — C — C — C — C'™

(j) H (j> H (J)

H H \

1 1 \

Polystyrene

c — c —

(Polymer)

1 1

(PS)

<t> H In

-</WN C C /WWWW\ Q Q

c Crosslinks

UP/PS

Crosslinked
polymer

resin
network

occur, and, finally, to a solid glassy vitreous state when numerous cross-links form and
tie together the polyester molecules in the resin system. This, in essence, is the chemi-
cal mechanism that characterizes the cure of a typical polyester resin.

In the commercial formulation of unsaturated polyester/styrene thermosetting FRP
resins, to make the resin more sag resistant when applied to vertical and more con-
toured surfaces, fumed silica is added to alter the rheology of the liquid resin. Another
additive involves using a wax material that serves as a surface active agent that allows
the resin to cure more evenly at its surface. Unsaturated polyester resin systems are by
far the most widely used FRP matrix resins because of their low cost and availability.
However, their use is being questioned because of environmental concerns. Styrene
monomer is quite odiferous, and questions are being raised regarding its human toxic-
ity after long-term process-operator exposure.

Chemistry of the epoxy resin system. Because of their inherent good adhesion to all
types of surfaces, epoxy resins are generally more difficult to work with than poly-
esters. However, epoxies have much better thermal properties and exhibit very low
shrinkage during cure. Their adhesive properties, while adding process difficulties,
serve to enhance the structural integrity of the fiber/resin composite material system.
Epoxies provide good adhesion of the resin matrix to the reinforcing fibers. The major
hardeners for epoxy resins are amines and anhydrides. The chemistry of these hard-
ener/curative systems is discussed next.

314 8 Plastics

Epoxy resins are characterized by the reaction of the epoxy group c c

known as the oxirane ring. Polymerization reactions proceed by the opening of this
oxirane ring to form a difunctional chemical-reacting specie similar to the unsaturated
C = C group in polyesters. Epoxy resins are low-molecular- weight polymers contain-
ing oxirane rings at each end of the chain. They are cured by adding a multifunctional
chemical to the mixture that serves to cross-link the system by an addition reaction
with the oxirane ring. The most common cross-linking agents for epoxies are the
amines. Many of the amines used to cure epoxies are liquids, which makes the amines
serve as reactive diluents. Such liquid material systems are also easily adaptable to
dual-component pumps and the mixing of resin during dispensing for processing. The
basic reaction between (primary) amine groups and the epoxy group is as follows:

RNH, + CH

1? —

— CH *~ RNH CH2 CH

?H o

/ \

CH?

CH + CH2 CH >- RN (CH2

RNH CH2 CH + CH2 CH ► RN (CH2 CHOH)

As shown, each of the two hydrogen atoms of the primary amine, RNH2, where R is a
generic unspecified organic grouping, is capable of reacting with one epoxide group. In
this chemical process, as with polyester resins, the epoxy polymer passes from a liquid to
a gel-rubber to the solid glassy state as the cross-linking reaction proceeds. During the lat-
ter stages of the reaction, the resultant OH groups that are formed in the amine reaction can
also react with epoxy groups and further increase the cross-link density of the polymer.

One advantage of amine-cured epoxy resins is that they can harden or cure at
room temperature. However, room temperature curing leads to polymers with low
temperature stability. Also, the moisture resistance of these epoxy resins is generally
low. Both temperature and moisture resistance can be improved by postcuring the
resins above 212°F (100°C). Here, the chemical cross-links of the resin are maximized
as complete reaction of all the epoxy groups is approached.

The reaction of anhydride curing agents with epoxy resins is more complex than
that of amine cures. With anhydrides, amine catalysts are required along with cures at
high temperature. During reaction, several competing reactions can take place. The
most important reactions are as follows:

1. Opening of the anhydride ring with the OH groups from the catalytically reacted
epoxy groups to form a carboxyl group:

o
c c^ ! c c o CH

O + HO CH *- :

— c — c i — c — c — OH

I V

anhydride epoxy resin reaction product

moiety

8.5 Fiber-Reinforced Polymeric Composites 315

2. Subsequent reaction of the carboxyl group with the epoxy group:

o o

■ o

C C O CH / \ C C O CH

i + CH2 — CH • — *~ i

C C OH C C O CH2 CH

O O OH

3. Epoxy groups, in turn, reacting with the formed OH groups:

! /°\

HC OH + CH2 CH ►■ =

= HC O CH? CH

Although all three reactions can occur, which of the three reactions predominates de-
pends on the reaction temperature.

Compared to amine cures, the pot life of anhydride cures is long, and the reaction
produces a low exotherm. Long-time, elevated-temperature cures up to 392°F (200°C)
are necessary if ultimate properties are desired. Overall, compared to amine-cured sys-
tems, anhydride cures result in much better chemical resistance for the final cured
resin product.

From a processing standpoint, the environmental and industrial hygiene aspects of
amine- or anhydride-cured epoxy resins are much better than the hygiene problems
associated with unsaturated polyester resin processing. In all cases, proper protective
clothing (coat, gloves, and goggles) must be worn while working with these resins.
Amine and anhydride chemicals are generally quite corrosive to the skin and may cause
dermatitis.

Forms of Composite Materials
and Fabrication Techniques

Discontinuous fiber reinforcement. The reaction injection molding (RIM) process in-
volves bringing together two components of a thermosetting polymeric resin system in
a mixing head and injecting the reacting mixture into a closed mold before reaction is
complete, as illustrated in Figure 8.25. The resin system then cures in the mold at a rel-
atively low pressure of 50 psi (345 kPa). The timing of the curing reaction is very im-
portant because the reaction must occur at the moment the mold cavity is filled. Close
process control is required. Because the process involves low-viscosity intermediates,
complex parts can be fabricated using the RIM method.

316

8 Plastics

FIGURE 8.25

The reaction injection
molding (RIM) process

Resin
component A

Resin
component B

High-
pressure
^ cylinder

Reinforcement (glass, fiber, or flake) can be added to one of the resin components
prior to mixing if increased flexural modulus, thermal stability, and, in some instances,
a special surface finish is desired in the final molded product. This process, reinforced
reaction injection molding (RRIM), is shown in Figure 8.26.

Structural reaction injection molding (SRIM) and resin transfer molding (RTM)
are similar to RRIM, except that the reinforcement is placed directly into the mold
prior to the injection of the resin. In SRIM, the reinforcement is typically a preform of
reinforcement fibers or mat of nonwoven fibers. In RTM, as shown in Figure 8.27, a
catalyzed resin is pumped directly into the mold cavity containing the reinforcement.

FIGURE 8.26

The reinforced reaction
injection molding
(RRIM) process

Reinforcement

Resin
component A

Resin
component B

High-pressure
cylinders

Mold

8.5 Fiber-Reinforced Polymeric Composites

317

FIGURE 8.27

The resin transfer
molding (RTM) process

Resin

Dry Reinforcement

The resin system is such that it cures without heat. The advantages of RTM are that,
because no mixing head is involved, a relatively low investment is needed for equip-
ment and tooling. Furthermore, large FRP parts and parts containing inserts and cores
can be fabricated using the RTM process. RIM, RRIM, SRIM, and RTM processing
are widely used in the automotive and aerospace industries.

Wet lay-up and vacuum bagging. Imbedding plies of glass, carbon, and/or polyaramid
plain-weave fabric or fibrous mat into an uncured liquid resin and allowing the liquid
resin to solidify (cure) while being constrained by a mold or form is a common pro-
cessing technique used in the pleasure boat building industry. A typical arrangement of
the plies used in this technique, called the wet lay-up process, is shown in Figure 8.28.
Related to this wet lay-up process is the vacuum bagging method of fabricating com-
posite parts and shapes. The principle of vacuum bagging is quite simple. The shape to be
fabricated is prepared by a room temperature wet lay-up procedure as just described. The
part to be fabricated is usually assembled over a form or shape of the desired (complex
and/or contoured) part. The assembly, like the lay-up arrangement shown in Figure 8.28,
is then placed in an airtight disposable plastic "bag" fitted with a vacuum tube fitting or
stem. If the air is sealed off and then evacuated from it, the bag will automatically close in
on the wet laid-up plies of fiber and liquid (uncured) resin and consolidate these plies by

FIGURE 8.28

Arrangement of plies in
wet lay-up assembly

u a d n u u u a u u u u o~q
n n n n n n w n n rrri n n rn

VBF - Impermeable vacuum bag film

B - Conformable nonwoven bleeder/breather fabric

P - Perforated release film

C - Fiber reinforced resin composite part

S - Pressure sensitive flexible sealant

318

8 Plastics

the action of atmospheric pressure. This composite assembly is then allowed to solidify
(cure) at room or elevated temperature. After this cure time, the vacuum bag, bleeder
ply, and resin-absorber material are removed from the assembly and discarded, leaving
the fabricated composite part ready for subsequent finishing or treatment.

A variation of the wet lay-up method is the spray-up process, where a spray gun si-
multaneously sprays catalyzed resin and chops continuous glass yarn into specific
lengths. As shown in Figure 8.29, chopped fibers enter the spray nozzel of the spray gun,
and the materials are comixed and sprayed onto an open-cavity mold. The mold usually
is faced with a smooth coating of already cured resin called a gel-coat or a thermoplas-
tic shell. This forms the outer surface of the structure being fabricated. When the
sprayed-on fiber-reinforced resin cures, the part is removed from the mold. The laminar
structure formed is composed of an aesthetically acceptable or otherwise finished outer
skin. Adhered to and backing up this skin is the cured fiber-reinforced resin. Open-mold
processing of this type is used extensively in bathtub and shower stall applications.

Unidirectional-fiber resin prepregs. Fiber-reinforced composite materials are com-
monly used in the form of a prepreg. Prepregs are typically side-by-side aligned fiber
yarns that have been impregnated by a B-staged resin matrix (meaning that it has been
deliberately partially cured). Unidirectional-fiber composite prepregs are commercially
available in the form of rolls, tapes, and sheets. One drawback is that these prepregs
must be kept frozen, below 32°F (0°C), for shipping and storage before use. They also
have a relatively short shelf life. If not properly stored, the B-stage resins will cure
slowly at room temperature, and their function will be destroyed.

Prepreg material is used to fabricate structures by plying together lay-ups of these
resin-impregnated unidirectional fibers. The lay-ups can be designed to have different
desired mechanical properties depending upon the geometrical arrangement or assem-
bly of the reinforcing fibers in the cured lay-up. Some typical unidirectional-fiber ply
arrangements are shown in Figure 8.30. Mechanically, these unidirectional (0°, 0°),
cross-ply (0°, 90°), and quasi-isotropic (0°, +45°, 90°, -45°, 0°) plied laminates will

FIGURE 8.29 ^^

The spray-up process f J \ —

8.5 Fiber-Reinforced Polymeric Composites

319

FIGURE 8.30

Various arrangements
of unidirectional-fiber
ply laminates

Unidirectional
(0°,0°)

Cross-ply
(0°,90°)

Quasi-isotropic
(0°,+45o,-45o,90°)

have planar anisotropic properties. Their flexural stiffness will always be higher in the
longitudinal direction of the fibers. Other forms of B-stage resin-impregnated fiber
forms are commercially available (e.g., fabrics and fibrous mats). The numerous B-
stage precomposite forms and types of fiber are all available to the composite materi-
als design engineer in the construction of a fiber-reinforced composite structure.

320 8 Plastics

Filament winding. Filament winding is a fiber-reinforced composite processing pro-
cedure commonly used to fabricate tubular (hollow) and cylindrical tank or bottle-
like structures. The apparatus used in the filament winding process is shown in Figure
8.31a. Basically, filamentary yarns are fed off a spool that is mounted on a creel. The
yarn is immersed in a catalyzed, but still liquid, resin bath, where the yarn is impreg-
nated with the resin. After squeezing out excess resin, the resin-impregnated yarn is
wound onto a rotating mandrel in a controlled and directed manner. A computer sys-
tem and control arm guide the yarn back and forth across the mandrel in a predeter-
mined pattern. The computer controls the type of wind pattern and the number of
layers of yarn filaments to be laid down on the mandrel surface. Two types of wind
patterns are possible: circumferential and helical, (as Figure 8.31b shows). In the cir-
cumferential or hoop wind, the yarn is wound in a continuous manner in close prox-
imity alongside itself. No crossover of the yarn occurs during the lay-down of a given
layer, and the lay-down pattern can thus be considered to be at a zero wind angle. The
wind proceeds back and forth across the mandrel until the desired number of layers is
accomplished. In the helical wind, the yarn is permitted to cross over itself and tra-
verses the length of the mandrel at a prescribed angle (e.g., 10°, 30°, 45°). Again, the
wind proceeds back and forth across the surface of the rotating mandrel until the de-
sired number of layers is formed.

In practice, combinations of hoop and helical wind are usually performed to fab-
ricate a part. The desired lay-down sequence is programmed on the computer. While
the desired (yarn) filament-wound resin composite is being formed on the mandrel,
heating lamps can be focused on the resin/fiber mass to affect partial cure of the resin
during this lay-down step. Once the desired winding pattern is completed, the man-
drel with its wound fiber/resin composite outer surface is left rotating. Rotation and
heat-lamp curing continue until the resin material is in a rigid enough state that the
rotation can stop and the cylindrical part and mandrel can be removed from the fila-
ment winding machine. Postcuring of the wound composite and mandrel can then be
accomplished by placing the assembly in an oven. After final curing, the mandrel is
removed from the core of the assembly. To facilitate this, the mandrel form is gener-
ally made with a slight taper along its length so that the mandrel can easily be
slipped out of an end, leaving the desired filamentary composite cylindrical "shell."
The composite part can then be machined and/or post-treated to the desired condition
or form.

Pultrusion processing. Pultrusion is a fiber-reinforced resin processing technique
that is readily adaptable to the continuous manufacture of constant cross-sectional lin-
ear composite shapes. Rods, I beams, angles, channels, and hollow tubes and pipes are
commonly produced by pultrusion processing. Pultrusion is a linear-oriented process-
ing method whereby yarns of reinforcing fiber are continuously immersed in and im-
pregnated with a catalyzed fluid resin. As the term pultrusion indicates, these
resin-impregnated continuous-fiber yarns are concurrently pulled through an elongated
heated die designed so that the fiber/resin composite mass exiting the die is sufficiently
cured and retains the cross-sectional shape of the die. The apparatus used in the pul-
trusion process is shown in Figure 8.32a. In practice, prescribed lengths of the formed

8.5 Fiber-Reinforced Polymeric Composites

321

FIGURE 8.31

The filament winding
process: (a) apparatus;
(b) wind patterns

Yarn spools
on a creel

Hoop wind

Mandrel

(a)

Mandrel

Hoop

Multiple helical

I-

Hoop and helical
(b)

Longitudinal

piece can be cut using an in-line cutoff wheel. Pultrusion is, therefore, adaptable to
low-cost, continuous production of constant cross-sectional composite shapes. The
process of pultrusion is critically controlled by the resin system used (e.g., unsaturated
polyester, epoxy, and vinyl ester resins), the temperature and temperature profile of the
heated die, and the rate of pulling through the die.

In the manufacture of pultruded shapes, such as those shown in Figure 8.32b, al-
though the core cross section of the composite is linear oriented, there is often a need
to wrap the outer surface of the composite with a webbing (nonwoven or woven tape)
of fibrous material. This serves to consolidate the pultruded shape and gives a much
more durable outer surface to the finished part. In this instance, thin veils of non-
woven or woven fabric tapes are fed into the entrance of the die along with the resin-
impregnated continuous-fiber yarns. This assembled mass of fibers and resin proceeds
to be pulled through the die as just described. The manufacture of hollow pultruded
shapes is common, and a special die is then required. A shaped insert or "torpedo" is
fitted at the die entrance and extends partway into it. The fluid resin-impregnated fibers
entering the die are now constrained by this center-core obstruction. With the proper

322

8 Plastics

FIGURE 8.32

The pultrusion process:
(a) apparatus; (b) cross-
sectional designs

Q^D

QZD

Puller
assembly

Cured

composite

material

Yarn spools
on a creel

(a)

^Z^p

Structural beams
shapes

////////

V/////A

Beams

(b)

Hollow
pipe and
column

pulling speed, die temperature profile, and catalyzed resin formulation, the shape of
the insert is retained as the desired hollow cross section of the part exits the die.

Engineering Design with Composite
Materials

In the development of commercial products, there are many considerations. The par-
ticular field of organic polymer engineering composites is no exception. It is impera-
tive for the engineer to have an integrated understanding of the design, materials
behavior, processing, and service performance behavior of composite materials in
order to develop a successful product. This integrated approach is diagrammed in Fig-
ure 8.33. Thus far, this section has reviewed some of the materials and processing as-
pects of fiber-reinforced organic polymer composites. The engineering design and final
application aspects of composite materials are covered next.

8.5 Fiber-Reinforced Polymeric Composites

323

FIGURE 8.33

Model of technical base
for engineered
composite materials
product development

First, however, in order to carry out an engineering design with organic poly-
mer composites, the engineer must recognize and understand their advantages and
limitations. Some of the advantages and disadvantages of carbon and polyaramid
fiber-reinforced polymeric composites are as follows.

Advantages of carbon fibers.

1. High stiffness-to-weight and strength-to-weight ratios

2. High compressive strength

3. Excellent fatigue resistance

4. Good wear resistance (self-lubricating) and low friction coefficient

5. Mechanical vibration damping ability better than metals

6. Excellent creep resistance

7. Corrosion resistance (when not in contact with metals)

8. Some (directional) electrical and thermal conductivity

9. Very low (to slightly negative) directional thermal expansion coefficient

10. Very broad engineering design versatility

11. Broad processing versatility

324 8 Plastics

12. Less energy required to manufacture engineering composite structures than to
fabricate with metals

Advantages of polyaramld fibers.

1. High stiffness-to-weight and strength-to-weight ratios

2. Excellent fatigue resistance

3. Excellent corrosion resistance

4. Good vibration damping properties

5. Better impact resistance than carbon fiber composites

6. Electrically insulating

Disadvantages.

1. Limited service temperature

2. Moisture sensitivity/swelling/distortion

3. Anisotropic properties

4. Low compression strength (polyaramid fiber)

5. Bimetallic corrosion (carbon fiber)

6. Relatively high cost of advanced fibers

With these features and limitations in mind, the design engineer can proceed to
create unique products. In the composites field, it is not appropriate to think only of
using composite materials as a materials replacement for existing products. New prod-
ucts that take advantage of the unique properties of composite materials can also be
conceived. Many of these new product concepts involve exploiting the remarkable
specific strengths and specific elastic moduli of the "advanced" fiber- reinforced com-
posites. The design engineer can choose from a multitude of reinforcing fiber types
and fiber geometry arrangements, as well as from a variety of matrix materials. He or
she has the freedom to mix in the design specification two or more diverse fiber types,
as well as the freedom to directionally place the reinforcing fibers. All these degrees
of freedom of choice are available so that the desired final component can be de-
signed and fabricated. Fiber-reinforced organic polymer engineering composites are,
therefore, capable of being used to create what can be referred to as integral design
engineering material structures (IDEMS). Through computer-aided design (CAD)
and finite-element stress analysis (FEM) techniques, new products are developed in
computer-model form. In creating the actual fabricated product, the other facets of the
integrated materials system manufacturing operation come into play (see Figure 8.33).
Some specific areas in the design of organic polymer composites are discussed next.

Cutting, hole drilling, and machining. Although composite parts and structures are
process molded to the near-finished state, machining, drilling, and trimming are often
required as final steps. Therefore, the assembly and the finishing of the fabricated part
are important in the creation of a final commercial product. There is always the possi-
bility of damaging the composite material in these finishing post-treatments. Delamina-

8.5 Fiber-Reinforced Polymeric Composites 325

tion, edge fraying, matrix cracking, or crazing leading to weak spots in the composite
material structure are all possible. Great care must be taken to maintain the compos-
ite's structural integrity and appearance.

Post-treatment of fiber-reinforced composites involves different tooling and pro-
cedures compared to what is done for metal or plastics. The abrasiveness of the fiber
and the possibility of fragmentation of the matrix resin are two factors to consider.
Composites are machined, cut, and trimmed more easily using processes similar to
grinding or abrasive cutting rather than conventional metal-cutting techniques. Also,
the method used is dictated by the type of fiber reinforcement. Glass fiber, carbon
fiber, and especially polyaramid fiber composites all require their own procedures.
For example, the cutting of polyaramid-fiber-reinforced composites is difficult be-
cause the fiber is so tough and does not cleave or cut in a brittle, fracture mode. Pol-
yaramid fibers undergo a process called fibrillation when "damaged" by the drilling,
cutting, or machining tool. Fuzzy edge cuts or fiber-filled drill holes are produced
when conventional machining and drilling tools are used. For polyaramid and for
other fiber-reinforced composite materials for that matter, water-jet cutting, laser cut-
ting, and diamond wire cutting are often used to achieve an acceptable edge profile
to the final machine-finished parts. For carbon-fiber-reinforced composites, the ther-
mal effects due to laser cutting, machining, and drilling can be a deterrent because
the carbon fibers are thermally conductive. A weakened, charred, heat-damaged zone
may surround the laser-cut edge. In summary, great care must be taken in the finish-
ing post-treatments of fiber-reinforced composite materials.

Adhesive and mechanical joining. Adhesives are the principal means of joining com-
posite materials to themselves and other materials of construction (metals, plastics,
wood). The reasons for this are numerous. Most importantly, adhesive bonds are
uniquely capable of distributing stress and can easily be joined into contoured shapes.
In mechanical joining, hole drilling is required, which can lead to delamination of the
composite and a stress concentration at the point of joining. The transfer of load from
one material to another without creating large stress concentrations is the ultimate goal
of materials joining. This can be achieved better by adhesive joining. Adhesives can
often be incorporated into the structural laminar shape being fabricated as a one-step
manufacturing process. Metal strips, layers, and/or fittings can easily be adhesively
"molded" in the manufactured structure during the composite processing stage (e.g.,
wet lay-up, filament winding, RIM, RRIM, and so on). Adhesive joining techniques
lend themselves to the creation of integrally designed structures as described previ-
ously. The various adhesive joint designs (lap shear, butt tensile, scarf joints, and so
on) were discussed in Chapter 4.

Structural adhesives are available in various forms and types. Most common are
the two-package epoxy resins. These formulated products are very similar to the epoxy
matrix resins used to create the fiber-reinforced composite materials themselves. Usu-
ally, these two-package products consist of part A, the epoxy resin prepolymer, and
part B, the curative (such as a primary amine or a polyamide/amine). Fillers, thicken-
ers, reactive diluents, tackifiers, and other processing aids such as silicone compounds
to improve the moisture durability of the adhesive are added to the final formulation.

326 8 Plastics

These two-part adhesives are mixed just before being applied to the surfaces of the
parts to be joined. The assembly is then placed in a compression mold, platen press, or
vacuum bagging arrangement, where heat may be applied to consolidate the layers
being joined and cure the adhesive. There are also some one-package paste adhesives
that are formulated with a latent curative; the curative reacts only at high temperature.

Another useful form of adhesive is the film adhesive. Film adhesives are used ex-
tensively in the aerospace industry. Here, adhesives exist in the form of sheets. These
sheets are malleable, are drapable, and can be cut using shears to the desired size and
shape. These films are then placed between the surfaces to be joined and are cured
under consolidation pressure and elevated temperature. Like the one-package adhe-
sives, these adhesives are formulated with a high-temperature-reacting latent curative.
Film adhesives, like the fiber-reinforced epoxy prepregs described earlier, must be
stored at low temperature and kept frozen until ready to use.

Also used in bonding composite materials are the acrylic adhesives. Acrylic ad-
hesives having different flexibilities are available. They cure at room temperature by a
free-radical polymerization reaction. One feature of acrylic adhesives is that cure can
be achieved by first coating the free-radical catalyst on the surfaces to be bonded. This
"catalyst-primed" surface can then be stored until it is ready for bonding. An uncat-
alyzed acrylic adhesive is then coated onto the catalyst-primed surface. The surfaces
to be joined are then mated under contact pressure and allowed to cure, undisturbed,
at room temperature. Acrylic adhesives can produce bonds that are very oil resistant.

Finally, it is important that the surfaces to be joined be clean and free of oils,
greases, and loose surface material layers. This is especially necessary when joining
composite materials to metals. Vapor degreasing, followed by a chemically alkaline
cleaning bath, is normally used for surface treating metals prior to adhesive bonding.

Sandwich-panel construction. Structural sandwich-panel construction consists of
face sheets made up of fiber-reinforced laminar composite material (or metal sheet)
adhesively bonded to both sides of a core material. This concept is illustrated in Fig-
ure 8.34. The principle behind sandwich construction is that the core material spaces
the facings away from the symmetric center of the panel. Therefore, in flexure, the
faces or outer skins of the panel are in tension or compression. This construction leads
to the reinforcement in the faces, which resists the bending of the panel. The columnar
strength of the honeycomb core material then provides the shear and compression
strength of this unique panel structure. Above all, the adhesive must be strong and
have a high enough shear and peel strength to withstand these shear stresses.

Sandwich construction leads to the use of panels that give the highest stiffness-to-
weight ratio of any material design. Sandwich-panel construction is used extensively
in aircraft and aerospace applications, where the core materials are generally honey-
combed in geometric shape. Honeycomb cores can be made of thin metal (aluminum
or titanium) or of fiber-reinforced resin sheet (e.g., thin sheet of resin-impregnated
glass, carbon, or polyaramid mat). The manufacture of honeycomb core by the expan-
sion process is shown in Figure 8.35. Manufacturing honeycomb core involves coating
discrete strips of adhesive onto sheets of core material. The specially coated core ma-
terial is then cured under compression to form a "log" or block of core material. The

8.5 Fiber-Reinforced Polymeric Composites

327

FIGURE 8.34

Structural sandwich
panel construction
(Courtesy Strong, A. B.
Fundamentals of
Composites
Manufacturing:
Materials, Methods,
and Applications.
Dearborn, Michigan:
Society of
Manufacturing
Engineers, 1989)

Face sheet

Honeycomb (Metal, composite, or paper)

Film adhesive

Face sheet

log must then be cut to the desired core height and subsequently expanded to form the
final core material. In some instances, the core material is dipped into a resin solution
so that the core structure can be consolidated or stiffened. Another method of making
honeycomb is the direct corrugation process. In some less demanding stiffness and
compression applications, a rigid foam core material can be used. Rigid foam and

FIGURE 8.35

Manufacture of
honeycomb core by the
expansion process

1 . Adhesive
strips are
coated onto
web.

Adhesive

2. Plies from step 1 are laid
to form a block.

3. Block is cured under
heat and compression.

II II
II

u u

II
II

1 2 3 4 5 6.

Plies-adhesive
coat index

4. Expansion leads to

formation of honeycomb cone.

328

8 Plastics

Kraft-paper-based honeycomb core panels are often used in truck cargo bed panels and
in door panels.

Painting and coating. Standard coating methods can be used for painting or coating
fiber-reinforced composite structures. In all cases, the surface of the composite must
be thoroughly prepared before the final coating is applied. Surface cleaning, sanding,
abrading, filling in surface grooves/blemishes, and a solvent wipe must be carried out
before the paint sealer and final paint finish are applied. Paint sealers and the final
paint coating must be dried/cured at temperatures below the cure temperature of the
composite part. Drying with infrared heaters can be troublesome as the heat location
and temperature cannot be properly controlled using this technique. Epoxy and
polyurethane-based surface coatings are especially useful in the painting of composite
structures.

REFERENCES

Kaverman, R. D. "Reinforced Plastics and Compos-
ites." In Michael L. Berins, ed., SPI Plastics Engi-
neering Handbook. New York: Van Nostrand
Reinhold, 1991.

Mayer, Rayner M. Design with Reinforced Plastics.
Design Council, K128 Haymarket, London SWIY
450. Bournemouth, England: Bourne Press Ltd.,
1995.

Schwartz, Mel M., ed. Composite Materials Hand-
book, 2nd ed. New York: McGraw-Hill, 1992.

Strong, A. B. Fundamentals of Composites Manu-
facturing: Materials, Methods, and Applications.
Dearborn, Michigan: Society of Manufacturing En-
gineers, 1989.

Review Questions

,v,

1. What are plastics and why are they called poly-
mers'?

2. What is a monomer"?

3. Are all polymers artificial? Give examples.

4. Why is the strength of polymers lower than that
of metals?

5. Why is the electrical conductivity of polymers
lower than that of metals?

6. When did polymers start to gain widespread ap-
plication and why?

7. How are polymers classified based on their
temperature characteristics?

8. What is meant by chemical families of poly-
mers? Give examples.

9. What are the main characteristics of a thermo-
plastic polymer?

10. Does a thermoplastic polymer have a fixed
melting temperature? Why?

11. What is meant by shaping memoryl

12. What are the main characteristics of a ther-
mosetting polymer?

13. How do molecules of a thermosetting polymer
differ from those of a thermoplastic polymer?

Chapter 8 Review Questions

329

14. Compare the properties of plastics with those of
metals. How do the differences affect the de-
sign of plastic products?

15. How can we have different polymers starting
from the same monomer?

16. List four polymers that belong to the ethenic
group. Discuss their properties and applica-
tions.

17. What are the main applications of polyacetals?

18. What is cellophane and how is it produced?

19. What is the major disadvantage of cellulose ni-
trate?

20. What are the major applications for cellulose
acetate?

21. What is the chief limitation of nylons?

22. What are the major characteristics of pheno-
lics?

23. How are polyimides manufactured?

24. List the common applications for epoxies.

25. Discuss the properties of polyurethanes and list
some of their applications.

26. What property characterizes silicones? Suggest
suitable applications to make use of that prop-
erty.

27. Explain how natural rubber is processed.

28. Why are additives compounded with polymers?

29. List some fillers. Why are they added to poly-
mers?

30. What happens when too much filler is added?

31. How does the addition of plasticizers affect the
properties of a polymer?

32. List some of the lubricants used when process-
ing polymers.

33. What are the mechanisms for coloring poly-
mers?

34. Are all polymers cast in the same manner?

35. What are the design features of parts produced
by blow molding?

36. Using sketches, explain the injection molding
process.

37. What is the chief limitation of injection mold-
ing?

38. What kinds of polymers are usually processed
by compression molding?

39. List some advantages of the compression mold-
ing process.

40. What is the main difference between compres-
sion molding and transfer molding?

41. Explain briefly the operating principles of rota-
tional molding.

42. List examples of plastic products that are man-
ufactured by extrusion.

43. What is the coextrusion process? Why is it used
in industry?

44. What are the design features of parts produced
by thermoforming? Give examples.

45. What are the products of the calendering
process?

46. What is the major problem experienced when
machining plastics?

47. Using sketches, explain the process of hot-plate
joining.

48. Describe thermal staking.

49. Explain how ultrasonics are employed in weld-
ing and assembling plastic parts.

50. Do all plastics render themselves suitable for
ultrasonic welding? Explain.

51. What are the basic components of ultrasonic
welding equipment?

52. Using sketches, show some designs of ultra-
sonic-welded joints. List the characteristics of
each.

53. Explain the sequence of operations involved in
open-mold processing of reinforced polymers.

54. What are the similarities and differences be-
tween extrusion and pultrusion of polymers?

55. What are the design features of parts manufac-
tured by filament winding?

330

8 Plastics

56. Explain briefly the nature of FRP composites.

57. How can you predict the properties of a com-
posite? Provide a quantitative equation.

58. List some of the fibers used as inforcement in
FRP composites.

59. Briefly discuss the various matrix resins for
FRP composites indicating their advantages,
disadvantages, and limitations.

60. Why is vacuum bagging used in the modified
version of the wet lay-up method?

61. What should we be careful about when using
fiber resin prepregs?

62. What are the advantages of sandwich panels?

Design Pxojects__

The current products of a company involve dif-
ferent fruit preserves in tin cans, each containing
8 ounces (about 250 g). The company uses
250,000 tin cans annually, and each costs 13
cents. Because their machines are almost obso-
lete and the cost of tin is rising every year, the
company is considering replacing the tin cans
with plastic containers. Design plastic containers
to serve this goal, taking into account the plastic-
processing method to be used. Also, make a fea-
sibility study for the project.

Design a plastic cup that has a capacity of 8
ounces (about 250 g) of water. Assume the an-
nual production volume is 20,000 pieces.

Design a high-quality plastic pitcher that has a
capacity of 32 ounces (about 1 kg) of liquid. As-

sume the annual production volume is 15,000
pieces.

Design a wheel for a bicycle so that it can be
produced by injection molding instead of sheet
metal forming. The diameter is 24 inches (600
mm), and a load of 100 pounds (about 45 kg) is
applied, through the axle, at its center. Assume
the annual production volume is 100,000 wheels.

A trash container that has a capacity of 1 cubic
foot (0.027 m3) is made of sheet metal and can
withstand an axial compressive load of 110
pounds (50 kg). Redesign it so that it can be
made of plastic. Assume the annual production
volume is 20,000 pieces.

Chapter 9

yslcs of

Metal Cutting

INTRODUCTION

Metal cutting can be defined as a process during which the shape and dimen-
sions of a workpiece are changed by removing some of its material in the form
of chips. The chips are separated from the workpiece by means of a cutting
tool that possesses a very high hardness compared with that of the workpiece,
as well as certain geometrical characteristics that depend upon the conditions
of the cutting operation. Among all of the manufacturing methods, metal cut-
ting, commonly called machining, is perhaps the most important. Forgings and
castings are subjected to subsequent machining operations to acquire the pre-
cise dimensions and surface finish required. Also, products can sometimes be
manufactured by machining stock materials like bars, plates, or structural sec-
tions.

Machining comprises a group of operations that involve seven basic chip-
producing processes: shaping, turning, milling, drilling, sawing, broaching, and
grinding. Although one or more of these metal-removal processes are performed
at some stage in the manufacture of the vast majority of industrial products, the
basis for all these processes (i.e., the mechanics of metal cutting) is yet not fully
or perfectly understood. This is certainly not due to the lack of research but
rather is caused by the extreme complexity of the problem. A wide variety of fac-
tors contribute to this complexity, including the large plastic strains and high
strain rates involved, the heat generated and high rise in temperature during ma-
chining, and, finally, the effect of variations in tool geometry and tool material. It
seems, therefore, realistic to try to simplify the cutting operation by eliminating

331

332

9 Physics of Metal Cutting

FIGURE 9.1

Two-dimensional cutting
using a prismatic,
wedge-shaped tool

Workpiece

as many of the independent variables as possible and making appropriately im-
plicit assumptions if an insight into this complicated process is to be gained. In
fact, we are going to take this approach in discussing the cutting tools and the
mechanics of chip formation. We are going to consider two-dimensional cutting,
in which a prismatic, wedge-shaped tool with a straight cutting edge is employed,
as shown in Figure 9.1, and the direction of motion of the tool (relative to the
workpiece) is perpendicular to its straight cutting edge. In reality, such condi-
tions resemble the case of machining a plate or the edge of a thin tube and are
referred to as orthogonal cutting.

UTTING ANGLES

Figure 9.2 clearly illustrates that the lower surface of the tool, called the flank, makes
an angle \j/ with the newly machined surface of the workpiece. This clearance angle is
essential for the elimination of friction between the flank and the newly machined sur-
face. As can also be seen in Figure 9.2, there is an angle a between the upper surface,
or face, of the tool along which chips flow and the plane perpendicular to the machined

FIGURE 9.2

Tool angles in two-
dimensional cutting

Tool angle

Clearance
angle

9.1 Cutting Angles

333

surface of the workpiece. It is easy to realize that the angle a indirectly specifies the
slope of the tool face. This angle is known as the rake angle and is necessary for shov-
eling the chips formed during machining operations. The resistance to the flow of the
removed chips depends mainly upon the value of the rake angle. As a consequence, the
quality of the machined surface also depends on the value of the rake angle. In addi-
tion to these two angles, there is the tool angle (or wedge angle), which is the angle
confined between the face and the flank of the tool. Note that the algebraic sum of the
rake, tool, and clearance angles is always equal to 90°. Therefore, it is sufficient to de-
fine only two of these three angles. In metal-cutting practice, the rake and clearance
angles are the ones that are defined.

As you may expect, the recommended values for the rake and clearance angles are
dependent upon the nature of the metal-cutting operation and the material of the work-
piece to be machined. The choice of proper values for these two angles results in the
following gains:

1. Improved quality of the machined surface

2. A decrease in the energy consumed during the machining operation (most of
which is converted into heat)

3. Longer tool life as a result of a decrease in the rate of tool wear because the
elapsed heat is reduced to minimum

Let us now consider how the mechanical properties of the workpiece material af-
fect the optimum value of the rake and clearance angles. Generally, soft, ductile met-
als require tools with larger positive rake angles to allow easy flow of the removed
chips on the tool face, as shown in Figure 9.3. In addition, the higher the ductility of
the workpiece material, the larger the tool clearance angle that is needed in order to re-
duce the part of the tool that will sink into the workpiece (i.e., reduce the area of con-
tact between the tool flank and the machined workpiece surface). On the other hand,
hard, brittle materials require tools with smaller or even negative rake angles in order
to increase the section of the tool subjected to the loading, thus enabling the tool to
withstand the high cutting forces that result. Figure 9.4 illustrates tools having zero and
negative rake angles required when machining hard, brittle alloys. In this case, the
clearance angle is usually taken as smaller than that recommended when machining
soft, ductile materials.

FIGURE 9.3

Positive rake angle
required when
machining soft, ductile
metals

Positive
rake angle

Workpiece

334

9 Physics of Metal Cutting

FIGURE 9.4

Zero and negative rake
angles required when
machining hard, brittle
materials

Zero
rake angle

Negative
rake angle.

Workpiece

Tool

Workpiece

9.2 CHIP FORMATION

Mechanics of Chip Formation

There was an early attempt by Reuleaux at the beginning of the twentieth century to
explain the mechanics of chip formation. He established a theory that gained popular-
ity for many years; it was based on assuming that a crack would be initiated ahead of
the cutting edge and would propagate in a fashion similar to that of the splitting of
wood fibers, as shown in Figure 9.5. Thanks to modern research that employed high-
speed photography and quick stopping devices capable of freezing the cutting action,
it was possible to gain a deeper insight into the process of chip formation. As a result,
Reuleaux's theory collapsed and proved to be a misconception; it has been found that
the operation of chip formation basically involves shearing of the workpiece material.
Let us now see, step by step, how that operation takes place.

The stages involved in chip removal are shown in Figure 9.6. When the tool is set
at a certain depth of cut (see Figure 9.6a) and is then pushed against the workpiece, the
cutting edge of the tool and the face start to penetrate the workpiece material. The sur-
face layer of the material is compressed; then pressure builds up and eventually ex-
ceeds the elastic limit of the material. As a result of the intense shear stress along the
plane N-N, called the shear plane, plastic deformation takes place, and the material of
the surface layer has no option but to flow along the face of the tool without being sep-
arated from the rest of the workpiece (see Figure 9.6b). With further pushing of the
tool, the ultimate tensile strength is exceeded, and a little piece of material (a chip) is

FIGURE 9.5

Reuleaux's
misconception of the
mechanics of chip
removal

Workpiece

9.2 Chip Formation

335

FIGURE 9.6

Stages in chip removal:
(a) tool set at a certain
depth of cut set; (b)
workpiece penetration;
(c) chip separation

Workpiece

Tool

Chip
separated

A new chip

being
generated

(b)

separated from the workpiece by slipping along the shear plane (see Figure 9.6c). This
sequence is repeated as long as the tool continues to be pushed against the workpiece,
and the second, third, and subsequent chips are accordingly separated.

Types of Chips

The type of chip produced during metal cutting depends upon the following factors:

1. The mechanical properties (mainly ductility) of the material being machined

2. The geometry of the cutting tool

3. The cutting conditions used (e.g., cutting speed) and the cross-sectional area of
the chip

Based on these factors, the generated chips may take one of the forms shown in Fig-
ure 9.7. Following is a discussion of each type of chip.

Continuous chip. When machining soft, ductile metals such as low-carbon steel, cop-
per, and aluminum at the recommended cutting speeds (which are high), plastic flow
predominates over shearing (i.e., plastic flow continues, and shearing of the chip never
takes place). Consequently, the chip takes the form of a continuous, twisted ribbon (see
Figure 9.7a). Because the energy consumed in plastically deforming the metal is even-
tually converted into heat, coolants and lubricants must be used to remove the gener-
ated heat and to reduce friction between the tool face and the hot, soft chip.

Discontinuous chips. When machining hard, brittle materials such as cast iron or
bronze, brittle failure takes place along the shear plane before any tangible plastic flow
occurs. Consequently, the chips take the form of discontinuous segments with irregu-
lar shape (see Figure 9.7b). As no plastic deformation is involved, there is no energy
to be converted into heat. Also, the period of time during which a chip remains in con-

FIGURE 9.7

Types of machining
chips: (a) continuous,
twisted ribbon; (b)
discontinuous, irregular
segments; (c) sheared,
short ribbons

^Sfflto%

(a)

(b)

(c)

336

Physics of Metal Cutting

tact with the face of the tool is short, and. therefore, the heat generated due to friction
is very small. As a result, the tool does not become hot, and lubricants and coolants are
not required.

Sheared chips. When machining semiductile materials with heavy cuts and at rela-
tively low cutting speeds, the resulting sheared chips have a shape that is midway be-
tween the segmented and the continuous chips (see Figure 9.7c). They are usually
short, twisted ribbons that break every now and then.

The Problem of the Built-Up Edge

When machining highly plastic, tough metals at high cutting speeds, the amount of
heat generated as a result of plastic deformation and friction between the chip and the
tool is large and results in the formation of a built-up edge, as shown in Figure 9.8. The
combination of the resulting elevated temperature with the high pressure at the tool
face causes localized welding of some of the chip material to the tool face (see Figure
9.8a). The welded material (chip segment) becomes an integral part of the cutting tool,
thus changing the values of the cutting angles. This certainly increases friction, lead-
ing to the buildup of layer upon layer of chip material. This newly formed false cut-
ting edge (see Figure 9.8b) is referred to as the built-up edge. The cutting forces also
increase, the built-up edge breaks down, and the fractured edges adhere to the ma-
chined surface (see Figure 9.8c). The harmful effects of the built-up edge are increased
tool wear and a very poorly machined workpiece. The manufacturing engineer must
choose the proper cutting conditions to avoid the formation of a continuous chip with
a built-up edge.

The Cutting Ratio

As can be seen in Figure 9.9. during a cutting operation, the workpiece material just
ahead of the tool is subjected to compression, and, therefore, the chip thickness be-
comes greater than the depth of cut. The ratio of t0/t is called the cutting ratio (rc) and

FIGURE 9.8

Stages in the formation
of the built-up edge: (a)
localized welding; (b)
false cutting edge; (c)
flawed surface

Built-up
edge

(a)

(b)

Broken chips

sticking to the

newly machined

surface

(c)

9.2 Chip Formation

337

FIGURE 9.9

Geometry of a chip with
respect to depth of cut

can be obtained as follows:

sin (|)

r _*o _ ts sin $

t ts cos (()) - a) cos (§ - a)

(9.1)

By employing trigonometry and carrying out simple mathematical manipulation, we
can obtain the following equation:

tan <b =

rc cos a
1 - rc sin a

(9.2)

Equation 9.2 is employed in obtaining the value of the shear angle § when the rake
angle a, the depth of cut, and the final thickness of the chips are known. In experi-
mental work, the chip thickness is either measured directly with the help of a ball-
ended micrometer or obtained from the weight of a known length of chip (of course,
the density and the width of the chip must also be known).

Let us now study the relationship between velocities. Considering the constancy
of mass and assuming the width of the chip to remain constant, it is easy to see that

V x tn = V, x t

V t c

In other words,

Vr = Vrr =

V sin (j)
cos (()) - a)

(9.3)

We can now draw the velocity triangle because we know the magnitudes and directions
of two velocities, V and Vc. The shear velocity, Vv, which is the velocity with which the
metal slides along the shear plane, can then be determined. Based on the velocity tri-
angle shown in Figure 9.10 and applying the sine rule, the following can be stated:

sin (90 - (j» + a) sin (90 - a) sin ty

338

9 Physics of Metal Cutting

FIGURE 9.10

Velocity triangle and
kinematics of the chip-
removal process

(90 - <t> + a)

(0 -a)

(90 - a)

This equation can also take the form

V Vs _ vc

cos ((}) - a) cos a sin <j)

Therefore,

cos a

V=V

cos ((() - a)

(9.4)

Shear Strain During Chip Formation

The value of the shear strain is an indication of the amount of deformation that the
metal undergoes during the process of chip formation. As can be seen in Figure 9.11,
the parallelogram abda' will take the shape abed' due to shearing. The shear strain can
be expressed as follows:

a'n d'n ., .

y = h = cot (J) + tan (<|) - a)

an an

(9.5)

The shear strain rate can be obtained from Equation 9.5 as follows:

a'n 1 d'n 1

y = x — + x —

an At an At

a'd' 1

x —

an At

FIGURE 9.11

Shear strain during chip
formation

Rake angle

Shear

strain,

Final shape of material
, just after deformation
^ y (broken line

Original shape of
material just before
machining (hatched)

9.3 Cutting Forces

339

But,

a'd'

At
Therefore,

= K

Y =

where an is the thickness of the shear zone. Experimental results have indicated that
the thickness of the shear zone is very small. Consequently, it can easily be concluded
that the process of chip formation takes place at an extremely high strain rate. This
finding is very important, especially for strain-rate-sensitive materials, where the
strength and ductility of the material are markedly affected.

UTTING FORCES

Theory of Ernst and Merchant

In order to simplify the problem, let us consider the two-dimensional, idealized cut-
ting model of continuous chip formation. In this case, all the forces lie in the same
plane and, therefore, form a coplanar system of forces. Walter Ernst and Eugene M.
Merchant, both eminent American manufacturing scientists, based their analysis of
this system of forces on the assumption that a chip acts as a rigid body in equilib-
rium under the forces acting across the chip-tool interface and the shear plane. As
Figure 9.12 shows, the cutting edge exerts a certain force upon the workpiece. The
magnitude of that force is dependent upon many factors, such as the workpiece ma-
terial, the conditions of cutting, and the values of the cutting angles.

FIGURE 9.12

Cutting force diagram
according to Ernst and
Merchant

340 9 Physics of Metal Cutting

By employing simple mechanics, the force can be resolved into two perpendicu-
lar components, Fc and F,. As can be seen in Figure 9.12, Fc acts in the direction of
tool travel and is referred to as the cutting component, whereas F, acts normal to that
direction and is known as the thrust component. The resultant tool force can alterna-
tively be resolved into another two perpendicular components, Fs and Fn. The first
component, Fs, acts along the shear plane and is referred to as the shearing force; the
second component, Fn, acts normal to it and causes compressive stress to act on the
shear plane. Again, at the chip-tool interface, the components of the resultant force
that acts on the chip are F and TV. Notice from the figure that F represents the fric-
tion force that resists the movement of the chip as it slides over the face of the tool,
while N is the normal force. The ratio between F and TV is actually the coefficient of
friction at the chip-tool interface. Because each two components are perpendicular, it
is clear from Euclidean geometry that the point of intersection of each two compo-
nents must lie on the circumference of the circle that has the resultant force as a di-
ameter. The cutting force diagram of Figure 9.12 lets us express Fs, Fn, F, and TV in
terms Fc and F, as follows:

Fs = Fc cos § - F, sin (J) (9.6)

Fn = Fc sin <J) + F, cos (J) (9.7)

F = Fc sin a + Ft cos a (9.8)

N = Fc cos a - F, sin a (9.9)

The preceding equations can be used to determine different unknown parameters
that affect the cutting operations. For instance, the coefficient of friction at the chip-
tool interface can be obtained as follows:

F F,. sin cl + F, cos a _i 0

u. = — = — - - = tan B

TV Fc cos a- Ft sin a

Dividing both the numerator and denominator by cos a, we obtain

F, + F,tana (9>10)

Fc - F, tan a

The shear force Fs is of particular importance as it is used for obtaining the mag-
nitude of the mean shear strength of the material along the shear plane and during the
cutting operation. This is equal to the mean shear stress acting through the shear plane
and can be computed as follows:

' A,

where As, the area of the shear plane, equals Achip/sm ((), where Achip is the cross-
sectional area of the chip. Therefore,

x _ [Fc cos (() - F, sin <t>]sin ())

•™chip

9.3 Cutting Forces 341

Experimental work has indicated that the mean shear stress, calculated from
Equation 9.11, is constant for a given metal over a wide variation in the cutting con-
ditions. This can be explained by the fact that the strain rate at which metal cutting
occurs is sufficiently high to be the only factor that affects the shear strength for a
given material. Therefore, the cutting speed, amount of strain, or temperature do not
have any appreciable effect on the value of the mean shear stress of the metal being
machined.

Ernst and Merchant extended their analysis and studied the relationship between
the shear angle and the cutting conditions. They suggested that the shear angle always
takes the value that reduces the total energy consumed in cutting to a minimum. Be-
cause the total work done in cutting is dependent upon and is a direct function of the
component Fc of the cutting force, they developed an expression for Fc in terms of (J)
and the constant properties of the workpiece material. Next, that expression was dif-
ferentiated with respect to <J) and then equated to zero in order to obtain the value § for
which Fc and, therefore, the energy consumed in cutting is a minimum. Following is
the mathematical treatment of this problem.

From Figure 9.12, we can see that

Fs = R cos (4> + p - a) (9.12)

Therefore,

cos (§ + p - a)
But,

Fs = xsAs = xs

'chip

sin §
Therefore,

R = T*AchiP x (9.13)

sin § cos (<|) + P - a)

Again, it can be seen from Figure 9. 1 2 that

Fc = /?cos(p-<x) (9.14)

Hence, from Equations 9.13 and 9.14,

F = TAhiD x cos (p - a) (9lS)

sin § cos ((() + P - a)

Differentiating Equation 9.15 with respect to ({) and equating the outcome to zero, we
obtain the condition that will make Fc minimal. This condition is given by the follow-
ing equation:

20 + P - a = ^ (9.16)

342 9 Physics of Metal Cutting

It was found that the theoretical value of ty obtained from Equation 9.16 agreed well
with the experimental results when cutting polymers, but this was not the case when
machining aluminum, copper, or steels.

Theory of Lee and Shaffer

The theory of American manufacturing scientists E. Lee and Bernard W. Shaffer is
based on applying the slip-line field theory to the two-dimensional metal-cutting prob-
lem. A further assumption is that the material behaves in a rigid, perfectly plastic man-
ner and obeys the von Mises yield criterion and its associated flow rule. After
constructing the slip-line field for that problem, it was not difficult for Lee and Shaf-
fer to obtain the relationship between the cutting parameters and the shear angle. The
result can be given by the following equation:

<|> + f3 - a = -j (9.17)

In fact, neither of the preceding theories quantitatively agrees with experimental re-
sults. However, the theories yield linear relationships between ()) and ((3 - a), which is
qualitatively in agreement with the experimental results.

Cutting Energy

We can see from the previous discussion that it is the component Fc that determines
the energy consumed during machining because it acts along the direction of relative
tool travel. The power consumption P,„ (i.e., the rate of energy consumption during
machining) can be obtained from the following equation:

Pm =FcxV (9.18)

where V is the cutting speed.

The rate of metal removal during machining Zm is also proportional to the cutting
speed and can be given by

Zm =A0xV (9.19)

where A0, the cross-sectional area of the uncut chip, equals t0 times the width of the
chip. Now, the energy consumed in removing a unit volume of metal can be obtained
from Equations 9.18 and 9.19 as follows:

n P,„ Fc x V Fc

P( = — = — = — (9.20)

Zm At) x V A„

In Equation 9.20, Pc, a parameter that indicates the efficiency of the process, is
commonly known as the specific cutting energy and also sometimes is called the
unit horsepower. Unfortunately, the specific cutting energy for a given metal is not
constant but rather varies considerably with the cutting conditions, as we will see
later.

9.4 Oblique Versus Orthogonal Cutting

343

BLIQUE VERSUS ORTHOGONAL CUTTING

Until now, we have simplified the metal-cutting process by considering only orthogo-
nal cutting. In this type of cutting, the cutting edge of the tool is normal to the direc-
tion of relative tool movement, as shown in Figure 9.13a. It is actually a
two-dimensional process in which each longitudinal section (i.e., parallel to the tool
travel) of the tool and chip is identical to any other longitudinal section of the tool and
chip. The cutting force is, therefore, also two-dimensional and can be resolved into two
components, both lying within the plane of the drawing. Although this approach facil-
itated the analysis of chip formation and the mechanics of metal cutting, it is seldom
used in practice because it applies only when turning the end face of a thin tube in a
direction parallel to its axis.

The more common type (or model) of cutting used in the various machining op-
erations is oblique cutting. In this case, the cutting edge of the tool is inclined to (i.e.,
not normal to) the relative tool travel, as can be seen in Figure 9.13b. It is a three-
dimensional problem in which the cutting force can be resolved into three perpendic-
ular components, as indicated in Figure 9.14. The magnitudes of these components can
be measured by means of a special apparatus that is mounted either in the workholder
or toolholder and is known as a dynamometer. As you may expect, the tool geometry

FIGURE 9.13

Types of cutting: (a)
orthogonal; (b) oblique

(a)

(b)

FIGURE 9.14

Components of the
three-dimensional
cutting force

Tool

344 9 Physics of Metal Cutting

is rather complicated and will be discussed later. For now, let us see the effect of each
of the cutting force components on the oblique cutting operations.

Forces in Oblique Cutting

Following is a discussion of the three components referred to as Fc, Ff, and Fr in Fig-
ure 9.14:

1. Fc is the cutting force and acts in the direction where the cutting action takes place.
It is the highest of the three components and results in 99 percent of the energy con-
sumed during the process. The horsepower due to this force, hpc, can be given by
the following equation:

FcxVc

hp-=l^ <9-21)

In Equation 9.21, Fc is in pounds and Vc is in feet per minute. Consequently, the
appropriate conversion factors must be used if the horsepower is to be obtained in
SI units.

2. Ff is the feed force (or longitudinal force in turning). The term feed means the
movement of the tool to regenerate the cutting path in order to obtain the machined
surface. This force amounts to only about 40 percent of the cutting force. The
horsepower required to feed the tool, hpf, can be given as follows:

Ffx Vf

hpf = t?, 7- (9.22)

Ff 550x60

The horsepower given by Equation 9.22 amounts to only 1 percent of the total
power consumed during cutting.

3. Fr is the thrust force (or radial force in turning) and acts in the direction of the
depth of cutting. This force is the smallest of the three components and amounts to
only 20 percent of the cutting force or, in other words, 50 percent of the feed force.
This component does not result in any power consumption as there is no tool move-
ment along the direction of the depth of cut.

These components of the cutting force are measured only in scientific metal-
cutting research. The manufacturing engineer is, however, interested in determining
beforehand the motor horsepower required to perform a certain job in order to be able
to choose the right machine for that job. Therefore, use is made of the concept of unit
horsepower, which was mentioned previously. Experimentally obtained values of unit
horsepower for various common materials are compiled in tables ready for use. The
total cutting horsepower can be obtained from the following equation:

hpc = unit hp x rate of metal removal x correction factor (9.23)

where the rate of metal removal is in cubic inches per minute and the correction fac-
tor is introduced to account for the tool geometry and and the variation in feed.

9.4 Oblique Versus Orthogonal Cutting

345

Table 9.1 indicates the unit horsepower values for various ferrous metals and al-
loys having different hardness numbers. Table 9.2 provides the unit horsepower values
for nonferrous metals and alloys. Figure 9.15a through c indicates the different correc-
tion factors for the unit horsepower to account for variations in the cutting conditions.

The cutting horsepower is not of practical importance by itself. Its significance is
that it is used in computing the motor horsepower. Obviously, the motor horsepower

TABLE 9.1

Unit horsepower values for ferrous metals and alloys

Brinnei Hardness Number

Ferrous

Metals and

Alloys

150-175

176-200

201-250

251-300

301-350

351-400

ANSI

1010-1025

.58

.67

—

1030-1055

.58

.67

.80

.96

—

1060-1095

—

.75

.88

1.0

—

1112-1120

.50

—

1314-1340

.42

.46

.50

—

1330-1350

—

.67

.75

.92

1.1

—

2015-2115

.67

—

2315-2335

.54

.58

.62

.75

.92

1.0

2340-2350

—

.50

.58

.70

.83

—

2512-2515

.50

.58

.67

.80

.92

—

3115-3130

.50

.58

.70

.83

1.0

3160-3450

—

.50

.62

.75

.87

1.0

4130-4345

—

.46

.58

.70

.83

1.0

4615-4820

.46

.50

.58

.70

.83

.87

5120-5150

.46

.50

.62

.75

.87

1.0

52100

—

.58

.67

.83

1.0

—

6115-6140

.46

.54

.67

.83

1.0

—

6145-6195

—

.70

.83

1.0

1.2

1.3

Plain cast iron

.30

.33

.42

.50

—

Alloy cast iron

.30

.42

.54

—

Malleable iron

.42

—

Cast steel

.62

.67

.80

—

Source: Turning Handbook of High-Efficiency Metal Cutting, 7950, courtesy Carboloy Inc., a Seco Tools Company.

346

9 Physics of Metal Cutting

TABLE 9.2

Unit horsepower values
for nonferrous metals
and alloys

Nonferrous Metals and Alloys

Properties

Unit Horsepower

Brass

Hard

0.83

Medium

0.50

Soft

0.33

Free machining

0.25

Bronze

Hard

0.83

Medium

0.50

Soft

0.33

Copper

Pure

0.90

Aluminum

Cast

0.25

Hard (rolled)

0.33

Monel

(rolled)

1.0

Zinc alloy

(die cast)

0.25

Source: Turning Handbook of High-Efficiency Metal Cutting, 1980, courtesy Carboloy Inc., A Seco Tools
Company.

has to be higher than the cutting horsepower as some power is lost in overcoming fric-
tion and inertia of the moving parts. The following equation can be used for calculat-
ing the motor horsepower:

hpm = hpc x

(9.24)

where r\ is the machine efficiency, which can be taken from Table 9.3.

The cutting horsepower is used not only in calculating the motor horsepower
but also for giving a fair estimate of the cutting force component Fc by using Equa-
tion 9.21. This force is very important when studying the vibrations associated with
metal cutting, as we will see later. The following example illustrates how to estimate
the cutting force component.

Example of Estimating Cutting
Force Component

During a turning operation, the metal-removal rate (M.R.R.) was found to be 3.6 cubic
inches per minute. Following are other data of the process:

Material:
Cutting speed:
Undeformed chip thickness:
Tool character:

Estimate the cutting force component Fc

ANSI 1055, HB 250

300 feet per minute

0.01 inch

0-7-7-7-15-15-1/32 (see Section 9.5)

9.4 Oblique Versus Orthogonal Cutting

347

FIGURE 9.15

Different correction
factors to account for
variations in the cutting
conditions: (a) cutting
speed; (b) chip
thickness; (c) rake
angle (Source: Turning
Handbook of High-
Efficiency Metal
Cutting, 1980, courtesy
Carboloy Inc., A Seco
Tools Company)

1.6
1.2
0.8
0.4

200 400 600 800
Cutting speed (SFPM)

(a)

0.0010.002 0.004 0.010 0.020 0.040 0.100
Undeformed chip thickness (in.)

(b)

1.4

1.2

1.0

0.8

0.6

-20 -10 0 +10 +20
True rake angle

Here is the solution to this example problem:

spindle hp = M.R.R. x unit hp x correction factor

The correction factor because of the cutting speed is 0.8, and the correction factor be-
cause of the undeformed chip thickness is 1 . The true rake angle is

tan a,™ = cos 15° tan 7° + sin 15° tan 0

TABLE 9.3

Typical overall machine

Type

Efficiency (%)

tool efficiencies (except

milling machines)

Direct-spindle drive

One-belt drive

Two-belt drive

Geared head

Source: Turning Handbook of High-Efficiency Metal
Cutting, 1980, courtesy Carboloy Inc., A Seco Tools
Company.

348

Physics of Metal Cutting

where atrue = 6° (see Section 9.5). The correction factor because of the true rake angle
is 0.83, and the unit hp is 0.8 from Table 9.1. Thus,

spindle hp = 3.6 x 0.8 x 0.8 x 1 x 0.83 = 1.9 hp

Fr x 300 ft/min.

hpc =

Therefore,

550 x 60

„ 1.9x550x60

Fc = — = 209 pounds

300 F

Note that the undeformed chip thickness equals feed (inches per revolution) times the
cosine of the side cutting-edge angle.

9.5 CUTTING TOOLS

Basic Geometry

In order for a tool to cut a material, it must have two important characteristics: First, it
must be harder than that material, and, second, it must possess certain geometrical
characteristics. The cutting tool geometry differs for different machining operations.
Nevertheless, it is always a matter of rake and clearance angles. Therefore, we are
going to limit our discussion, at the moment, to single-point tools for the sake of sim-
plicity. Other types of tools will be considered when we cover the various machining
operations.

As can be seen in Figure 9.16, the geometry of a single-point cutting tool can be
adequately described by six cutting angles. These can be shown more clearly by pro-
jecting them on three perpendicular planes using orthogonal projection, as is done in
Figure 9.17. Let us now consider the definition of each of the six angles.

Side cutting-edge angle. The side cutting-edge angle (SCEA) is usually referred to as
the lead angle. It is the angle enclosed between the side cutting edge and the longitu-
dinal direction of the tool. The value of this angle varies between 0° and 90°, depend-
ing upon the machinability, rigidity, and, sometimes, the shape of the workpiece (e.g.,

FIGURE 9.16

Geometry of a single-
point cutting tool

End relief

9.5 Cutting Tools

349

FIGURE 9.17

Orthogonal projection
of the cutting angles of
a single-point tool and
tool character

ECEA 20°

Nose radius

SCEA 15°
Top view

Side rake 8°

Back rake 2°

Side relief 6

Side view
(dotted lines are not shown)

[* End relief 6°
Front view

Tool character
2° 8° 6° 6

Back rake-
Side rake-

End relief-
Side relief -
ECEA

20° 15° ±\n.

SCEA-

Nose radius

a 90° shoulder must be produced by a 0° SCEA). As this angle increases from 0° to
15°, the power consumption during cutting decreases. However, there is a limit for in-
creasing the SCEA, beyond which excessive vibrations take place because of the large
tool-workpiece interface. On the other hand, if the angle were taken as 0°, the full cut-
ting edge would start to cut the workpiece at once, causing an initial shock. Usually,
the recommended value for the lead angle should range between 15° and 30°.

End cutting-edge angle. The end cutting-edge angle (ECEA) serves to eliminate rub-
bing between the end cutting edge and the machined surface of the workpiece. Al-
though this angle takes values in the range of 5° to 30°, commonly recommended
values are 8° to 15°.

Side relief and end relief angles. Side and end relief angles serve to eliminate rub-
bing between the workpiece and the side and end flank, respectively. Usually, the value
of each of these angles ranges between 5° and 15°.

Back and side rake angles. Back and side rake angles determine the direction of flow
of the chips onto the face of the tool. Rake angles can be positive, negative, or zero. It
is the side rake angle that has the dominant influence on cutting. Its value usually
varies between 0° and 15°, whereas the back rake angle is usually taken as 0°.

Another useful term in metal cutting is the true rake angle, which is confined be-
tween the line of major inclination within the face of the tool and a horizontal plane.
It determines the actual flow of chips across the face of the tool and can be obtained

350 9 Physics of Metal Cutting

from the following equation:

true rake angle = tan" '(tan a sin X + tan f3 cos X) (9.25)

where: a is the back rake angle
(3 is the side rake angle
X is the lead angle (SCEA)

As previously mentioned, the true rake angle has a marked effect on the unit horse-
power for a given workpiece material, and a correction factor has to be used when cal-
culating the power in order to account for variations in the true rake angle.

Tool character. The tool angles are usually specified by a standard abbreviation sys-
tem called the tool character, or the tool signature. As also illustrated in Figure 9.17,
the tool angles are always given in a certain order: back rake, side rake, end relief, side
relief, ECEA, and SCEA, followed by the nose radius of the tool.

Cutting Tool Materials

Cutting tools must possess certain mechanical properties in order to function ade-
quately during the cutting operations. These properties include high hardness and the
ability to retain it even at the elevated temperatures generated during cutting, as well
as toughness, creep and abrasion resistance, and the ability to withstand high bearing
pressures. Cutting materials differ in the degree to which they possess each of these
mechanical properties. Therefore, a cutting material is selected to suit the cutting con-
ditions (i.e., the workpiece material, cutting speed or production rate, coolants used,
and so on). Following is a survey of the commonly used cutting tool materials.

Plain-carbon steel. Plain-carbon steel contains from 0.8 to 1.4 percent carbon, has no
additives, and is subjected to heat treatment to increase its hardness. Plain-carbon steel
is suitable only when making hand tools or when soft metals are machined at low cut-
ting speeds as it cannot retain its hardness at temperatures above 600°F (300°C) due to
tempering action.

Alloy steel. The carbon content of alloy steel is similar to that of plain-carbon steel,
but it contains alloying elements (in limited amounts). Tools made of alloy steel must
be heat treated and are used only when machining is carried out at low cutting speeds.
The temperature generated as a result of cutting should not exceed 600°F (300°C) to
avoid any tempering action.

High-speed steel. High-speed steel (HSS) is a kind of alloy steel that contains a cer-
tain percentage of alloying elements, such as tungsten (18 percent), chromium (4 per-
cent), molybdenum, vanadium, and cobalt. High-speed steel is heat treated by heating
(at two stages), cooling by employing a stream of air, and then tempering it. Tools
made of HSS can retain their hardness at elevated temperatures up to 1 100°F (600°C).
These tools are used when relatively high cutting speeds are required. Single-point
tools, twist drills, and milling cutters are generally made of high-speed steel, except
when these tools are required for high-production machining.

9.5 Cutting Tools 351

Cast hard alloys. Cast hard alloys can be either ferrous or nonferrous and contain
about 3 percent carbon, which, in turn, reacts with the metals to form very hard car-
bides. The carbides retain their hardness even at a temperature of about 1650°F
(900°C). Because such a material cannot be worked or machined, it is cast in ceramic
molds to take the form of tips that are mounted onto holders by brazing or by being
mechanically fastened.

Sintered cemented-carbide tips. Sintered cemented carbide was developed to elimi-
nate the main disadvantage of the hard cast alloys: brittleness. Originally, the compo-
sition of this material involved about 82 percent very hard tungsten carbide particles
and 18 percent cobalt as a binder. Sintered cemented carbides are always molded to
shape by the powder metallurgy technique (i.e., pressing and sintering, as was ex-
plained in Chapter 7). As it is impossible to manufacture the entire tool out of ce-
mented carbide because of the strength consideration, only tips are made of this
material; these tips are brazed or mechanically fastened to steel shanks that have the
required cutting angles.

Cemented carbides used to be referred to as Widia, taken from the German ex-
pression "Wie Diamant," meaning diamondlike, because they possess extremely high
hardness, reaching about 90 Re, and they retain such hardness even at temperatures of
up to 1850°F (1000°C). Recent developments involve employing combinations of
tungsten, titanium, and tantalum carbides with cobalt or nickel alloy as binders. The re-
sult is characterized by its low coefficient of friction and high abrasion resistance.
Tools with cemented-carbide tips are recommended whenever the cutting speeds re-
quired or the feed rates are high and are, therefore, commonly used in mass produc-
tion. Recently, carbide tips have been coated with nitrites or oxides to increase their
wear resistance and service life.

Ceramic tips. Ceramic tips consist basically of very fine alumina powder, A1203,
which is molded by pressing and sintering. Ceramics have almost the same hardness
as cemented carbides, but they can retain that hardness up to a temperature of 2200°F
(1100°C) and have a very low coefficient of thermal conductivity. Such properties
allow for cutting to be performed at speeds that range from two to three times the cut-
ting speed used when carbide tips are employed. Ceramic tips are also characterized by
their superior resistance to wear and to the formation of crater cavities. They require
no coolants. Their toughness and bending strength are low, which must be added to
their sensitivity to creep loading and vibration. Therefore, ceramic tips are recom-
mended only for finishing operations (small depth of cut) at extremely high cutting
speeds of up to 180 feet per minute (600 m/min.). Following are the three common
types of ceramic tips:

1. Oxide tips, consisting mainly of aluminum oxide, have a white color with some
pink or yellow tint.

2. Cermet tips, including alumina and some metals such as titanium or molybdenum,
are dark gray in color.

3. Tips that consist of both oxides as well as carbides are black in color.

352 9 Physics of Metal Cutting

Ceramic tips should not be used for machining aluminum because of their affinity to
oxygen.

Diamond. Diamond pieces are fixed to steel shanks and are used in precision cutting
operations. They are recommended for machining aluminum, magnesium, titanium,
bronze, rubber, and polymer. When machining metallic materials, a mirror finish can
be obtained.

Tool Wear

There are two interrelated causes for tool wear: mechanical abrasion and thermal ero-
sion. Although these two actions take place simultaneously, the role of each varies for
various cutting conditions. Mechanical wear is dominant when low cutting speeds are
used or when the workpiece possesses high machinability. Thermal wear prevails when
high cutting speeds are used with workpieces having low machinability. Thermal wear
is due to diffusion, oxidation, and the fact that the mechanical properties of the tool
change as a result of the high temperature generated during the cutting operation.

The face of the cutting tool is subjected to friction caused by the fast relative
motion of the generated chips onto its surface. Similarly, the flanks are also subjected
to friction as a result of rubbing by the workpiece. Although the tool is harder than
the workpiece, friction and wear will take place and will not be evenly distributed
over the face of the tool. Wear is localized in the vicinity of the cutting edge and re-
sults in the formation of a crater. There are different kinds of tool wear:

1. Flank wear

2. Wear of the face that comes in contact with the removed chip

3. Wear of the cutting edge itself

4. Wear of the nose

5. Wear and formation of a crater

6. Cracks in the cutting edges occurring during interrupted machining operations
such as millings

Tool Life

Tool life is defined as the length of actual machining time beginning at the moment
when a just-ground tool is used and ending at the moment when the machining opera-
tion is stopped because of the poor performance of that tool. Different criteria can be
used to judge the moment at which the machining operation should be stopped. It is
common to consider the tool life as over when the flank wear reaches a certain amount
(measured as the length along the surface generated due to abrasion starting from the
tip). This maximum permissible flank wear is taken as 0.062 inch (1.58 mm) in the
case of high-speed steel tools and 0.03 inch (0.76 mm) for carbide tools.

The tool life is affected by several variables, the important ones being cutting
speed, feed, and the coolants used. The effect of these variables can be determined ex-
perimentally and then represented graphically for practical use. It was found by Fred-
erick W. Taylor that the relationship between tool life and cutting speed is exponential.
It can, therefore, be plotted on a logarithmic scale so that it takes the form of a straight

9.6 Machinability

353

FIGURE 9.18

Relationship between
tool life and cutting
speed on a log-log
scale

Tool life, min. (log scale)

line, as shown in Figure 9.18. In fact, this was the basis for establishing an empirical
formula that correlates tool life with cutting speed. A correction factor is also intro-
duced into the formula to account for the effects of other variables. The original for-
mula had the following form:

VTn = c

(9.26)

where: n is a constant that depends upon the tool material (0. 1 for HSS, 0.20 for
carbides, and 0.5 for ceramic tools)

c is a constant that depends upon the cutting conditions (e.g., feed)

T is the tool life measured in minutes

V is the cutting speed in feet per minute

Equation 9.26 is very useful in obtaining the tool life for any cutting speed if the tool
life is known at any other cutting speed.

9.6 MACHINABILITY

Machinability Defined

Machinability is a property characterizing the material of the workpiece: It is the ease
with which that material can be machined. In order to express machinability in a quan-
titative manner, one of the following methods is used:

1. The maximum possible rate of chip removal

2. Surface finish of the machined workpiece

3. Tool life

4. Energy required to accomplish the cutting operation

It is clear that the tool life is the most important of these criteria as it plays an impor-
tant role in maximizing the production while minimizing the production cost. More-
over, criteria such as surface finish and machining precision depend upon many
factors, such as the sharpness of the cutting edge, the rigidity of the tool, and the pos-

354

Physics of Metal Cutting

TABLE 9.4

Machinability indices
for some metals and
alloys (using carbide
tools)

Machinability

Metal or Alloy

Index (%)

Steel SAE 1020 (annealed)

Steel SAE A2340

Cast iron

70-80

Stainless steel 18-8 (austenitic)

Tool steel (low tungsten, chrome, and carbon)

Copper

Brass

180

Aluminum alloy

300 and above

sibility of formation of a built-up edge. As a consequence, it is the tool life that is most
suitable as a criterion of machinability.

Machinability Index

Because machinability cannot be expressed in an absolute manner, it is appropriate to
take a highly machinable metal as a reference and express the machinability of any
other ferrous metal as a percentage of that of the reference metal. The reference metal
chosen was steel SAE-AISI 1112 because of its superior machinability, which exceeds
that for any other steel. Such steel is usually referred to as free cutting steel. The
machinability index can now be given:

machinability index =

cutting speed of metal for tool life of 20 minutes
cutting speed of steel SAE 1 1 1 2 for tool life of 20 minutes

x 100 (9.27)

Table 9.4 indicates the machinability index for some commonly used metals and
alloys.

JUTTING FLUIDS

Necessary Characteristics

As previously mentioned, the process of metal cutting results in the generation of a
large amount of heat and a localized increase in the temperature of the cutting tool.
This effect is particularly evident when machining ductile metals. Accordingly,
coolants are required to remove any generated heat, to lower the temperature of the
cutting tool, and, consequently, to increase the tool service life. In order to fulfill such
conditions and function properly, a cutting fluid must possess certain characteristics:

1. The cutting fluid must possess suitable chemical properties (i.e., to be appropriate
from the point of view of chemistry), must not react with the workpiece material
or cause corrosion in any component of the machine tool, and should not promote

9.7 Cutting Fluids 355

the formation of rust or spoil the lubricating oil of the machine bearing and slides
whenever it comes in contact with that oil.

2. The cutting fluid must be chemically stable (i.e., must not change its properties
with time).

3. No poisonous gases or fumes should evolve during machining so that there is no
possibility of problems regarding the safety or health of the workers.

4. The lubricating and cooling properties of the cutting fluid must be superior.

5. The fluid used should be cheap and should be recycled by a simple filtration
process.

Types of Cutting Fluids

The following discussion involves the different kinds of cutting fluids that are used in
industry to satisfy the preceding requirements.

Pure oils. Mineral oils such as kerosene or polar organic oils such as sperm oil, lin-
seed oil, or turpentine can be used as cutting fluids. The application of pure mineral
oils is permissible only when machining metals with high machinability, such as free
cutting steel, brass, and aluminum. This is a consequence of their poor lubricating and
cooling properties. Although the polar organic oils possess good lubricating and cool-
ing properties, they are prone to oxidation, give off unpleasant odors, and tend to gum.

Mixed oils. Mineral oils are mixed with polar organic oils to obtain the advantages of
both constituents. In some cases, sulfur or chlorine is added to enable the lubricant to
adhere to the tool face, giving a film of lubricant that is tougher and more stable. The
oils are then referred to as sulfurized or chlorinated oils. The chlorinated oils have the
disadvantage of the possible emission of chlorine gas during the machining operation.

Soluble oils. Soluble oils are sometimes called water-miscible fluids or emulsifiable
oils. By blending oil with water and some emulsifying agents, soapy or milky mix-
tures can be obtained. These liquids have superior cooling properties and are recom-
mended for machining operations requiring high speeds and low pressures.
Sometimes, extreme-pressure additives are blended with the mixtures to produce
emulsions with superior lubricating properties.

Water solutions. A solution of sodium nitrate and trinolamine in water can be em-
ployed as a cutting fluid. Caustic soda is also used, provided that the concentration
does not exceed 5 percent. If the concentration of the solution exceeds this limit, the
paint of the machine and the lubricating oil of the slides may be affected.

Synthetic fluids. Synthetic fluids can be diluted with water to give a mixture that
varies in appearance from clear to translucent. Extreme-pressure additives like sulfur
or chlorine can be added to the mixture so that it can be used for difficult machining
operations.

356 9 Physics of Metal Cutting

HATTER PHENOMENON

When we feel cold in winter, our jaws and teeth may start to chatter. A similar phe-
nomenon occurs when the cutting tool and workpiece are exposed to certain unfavor-
able cutting conditions and dynamic characteristics of the machine tool structure. The
analysis of this chatter phenomenon is an extremely complex task. However, thanks to
the work of the late Professor Stephen A. Tobias of the University of Birmingham in
England, we are able to understand how vibrations of the cutting tool initiate and how
they can be minimized. Left without remedy, these vibrations result in breakage of the
cutting tool (especially if it is ceramic or carbide) and poor surface quality. They may
also cause breakage of the entire machine tool. Two basic types of vibrations are gen-
erated during machining: forced vibrations and self-excited vibrations.

Forced vibrations take place as a result of periodic force applied within the ma-
chine tool structure. This force can be due to an imbalance in any of the machine tool
components or interrupted cutting action, such as milling, in which there is a periodic
engagement and disengagement between the cutting edges and the workpiece. The fre-
quency of these forced vibrations must not be allowed to come close to the natural fre-
quency of the machine tool system or any of its components; otherwise, resonance
(vibrations with extremely high amplitude) takes place. The remedy in this case is to
try to identify any possible source for the imbalance of the machine tool components
and eliminate it. In milling machines, the stiffness and the damping characteristics of
the machine tool are controlled so as to keep the forcing frequency away from the nat-
ural frequency of any component and/or the natural frequency of the system.

Self-excited vibrations, or chatter, occur when an unexpected disturbing force,
such as a hard spot in the workpiece material or sticking friction at the chip-tool inter-
face, causes the cutting tool to vibrate at a frequency near the natural frequency of the
machine tool. As a result, resonance takes place, and a minimum excitation produces
an extremely large amplitude. Such conditions drastically reduce tool life, result in
poor surface quality, and may cause damage to either the workpiece or the machine
tool or both. This unfavorable condition can be eliminated, or at least reduced, by con-
trolling the stiffness and the damping characteristics of the system. This is usually
achieved by selecting the proper material for the machine bed (cast iron has better
damping characteristics than steel), by employing dry-bolted joints as energy dissipa-
tors where the vibration energy is absorbed in friction, or by using external dampers or
absorbers. Advanced research carried out at the University of Birmingham in England
indicated the potentials of employing layers of composites as a means to safeguard
against the occurrence of chatter.

[CONOMICS OF METAL CUTTING

Our goal now is to find out the operating conditions (mainly the cutting speed) that
maximize the metal-removal rate or the tool life. These two variables are in opposition
to each other; a higher metal-removal rate results in a shorter tool life. Therefore, some

9.9 Economics of Metal Cutting

357

FIGURE 9.19

Relationship between
cost per piece and
cutting speed

Tool-change
cost

Optimum cutting

speed for minimum

cost/piece

Cutting speed

trade-off or balance must be made in order to achieve either minimum machining cost
per piece or maximum production rate, whichever is necessitated by the production

requirements.

Figure 9.19 indicates how to construct the relationship between the cutting
speed and the total cost per piece for a simple turning operation. The total cost is
composed of four components: machining cost, idle-time (nonproductive) cost, tool
cost, and tool-change cost. An increase in cutting speed obviously results in a re-
duction in machining time and, therefore, lower machining cost. This is accompa-
nied by a reduction in tool life, thus increasing tool and tool-change costs. As can
be seen in Figure 9.19, the curve of the cost per piece versus the cutting speed has
a minimum that corresponds to the optimum cutting speed for the minimum cost

per piece.

The relationship between the production time per piece and the cutting speed can
be constructed in the same manner, as shown in Figure 9.20. There is also a minimum
for this curve that corresponds to the optimum cutting speed for the maximum pro-
ductivity (minimum time per piece). Usually, this value is higher than the maximum
economy speed given in Figure 9.19. Obviously, a cutting speed between these two
limits (and depending upon the goals to be achieved) is recommended.

358

9 Physics of Metal Cutting

FIGURE 9.20

Relationship between
production time per
piece and cutting speed

Tool -change
time per piece

Idle time

.Optimum cutting speed for
j maximum production

Cutting speed

Review Questions

1. How can the complex process of metal cutting
be approached?

2. Define the rake angle and the clearance angle
in two-dimensional cutting.

3. Why are the angles in Question 2 required?

4. What is the upper surface of the tool called?

5. What is the lower surface of the tool called?

6. What are the cutting variables that affect the
values of the rake and clearance angles?

7. List some drawbacks if the cutting angles are
not properly chosen.

8. When should the rake angle be taken as a posi-
tive value?

9. When should the rake angle be taken as a nega-
tive value?

10. Can orthogonal cutting actually take place?
Explain.

11. Use sketches to explain the stages involved in
the formation of chips during machining.

12. Use sketches to illustrate the different types of
machining chips and explain when and why we
can expect to have each of these types.

13. Explain the stages involved in the formation of
the built-up edge.

14. Does the built-up edge have useful or harmful
effects?

15. What is meant by the shear angle?

16. What is meant by the cutting ratio?

17. Derive an expression for the shear strain that
takes place during orthogonal cutting.

18. Draw a sketch of the cutting force diagram pro-
posed by Ernst and Merchant.

19. How can the relationship between the shear and
rake angles be expressed according to Ernst and
Merchant?

20. On what basis have Lee and Shaffer developed
their theory?

Chapter 9 Problems

359

21.
22.
23.

24.
25.

26.
27.

28.
29.

Derive an expression for the specific energy
during two-dimensional cutting.

Illustrate the difference between orthogonal and
oblique cutting.

What are the components of the cutting force in
oblique cutting? How do you compare their
magnitudes with each other?

Define the unit horsepower.

Describe fully the geometry of single-point cut-
ting tools.

Explain the effect of each of the cutting angles
in oblique cutting on the mechanics of the
process.

List the different cutting tool materials and enu-
merate the advantages, disadvantages, and ap-
plications of each.

What are the two main causes for tool wear?

List the different kinds of tool wear.

30. Define tool life.

31. What is the relationship between tool life and
cutting speed?

32. Define machinability and explain how it is
quantitatively expressed by the machinability
index.

33. What are the necessary characteristics of cut-
ting fluids?

34. List the different types of cutting fluids and
provide the advantages and limitations of each.

35. What are the causes for forced vibrations dur-
ing machining?

36. How can forced vibrations be minimized?

37. What is chatter and why does it occur?

38. How can we eliminate chatter?

39. What trouble can vibrations cause during ma-
chining?

40. Use sketches to explain how the value of
the optimum cutting speed can be obtained
for maximum economy and for maximum
productivity.

Problems

•o,

In a turning operation, the diameter of the work-
piece is 2 inches (50 mm), and it rotates at 360
revolutions per minute. How long will a carbide
tool last (n = 0.3) under such conditions if an
identical carbide tool lasted for 1 minute when
used at 1000 feet per minute (305.0 m/min.)?

Determine the increase in the tool life of a car-
bide tip as a result of a decrease in the cutting
speed of 25, 50, and 75 percent.
When turning a thin tube at its edge, the follow-
ing conditions were observed:

Depth of cut:
Chip thickness:
Back rake angle:
Cutting speed:

0.125 inch

0.15 inch

8°

300 ft/min.

Calculate the

a. Cutting ratio

b. Shear angle

c. Chip velocity

A geared-head lathe is employed for machining
steel AISI 1055, BHN 250. The cutting speed is
400 feet per minute, and the rate of metal re-
moval is 2.4 cubic inches per minute. If the tool
used has the character 0-7-7-7-15-15-1/32, esti-
mate the following:

The energy consumed in machining per unit
time

lime

The power required at the motor

The tangential component of the cutting

force

360

Physics of Metal Cutting

Neglect the correction factor for the undeformed
chip thickness.

5. A 5-hp, 2-V, belt-driven lathe is to be used for
machining brass under the following conditions:

Cutting speed:

Rate of metal removal:

600 ft/min.
7.2 in.7min

SCEA of the tool:

30°

Neglect the effect of chip thickness. Does this
lathe have enough power for the required job?

Design Prpiecl„

Prepare a computer program that determines the optimum cutting speed that results in
maximum productivity. The program should be interactive, the input being workpiece
material, tool material, and depth of cut. Assume the time for changing the tool is 60
seconds and the time to return the tool to the beginning of the cut is 20 seconds. Take
the workpiece material to be

a. Steel 1020

b. Brass

c. Aluminum

d. Stainless steel

Chapter 10

achlnlng
Metals

INTRODUCTION

This chapter will focus on the technological aspects of the different machining
operations, as well as the design features of the various machine tools em-
ployed to perform those operations. In addition, the different shapes and
geometries produced by each operation, the tools used, and the work-holding
devices will be covered. Special attention will be given to the required workshop
calculations that are aimed at estimating machining parameters such as cut-
ting speeds and feeds, metal-removal rate, and machining time.

Machine tools are designed to drive the cutting tool in order to produce the
desired machined surface. For such a goal to be accomplished, a machine tool
must include appropriate elements and mechanisms capable of generating the
following motions:

1. A relative motion between the cutting tool and the workpiece in the direc-
tion of cutting

2. A motion that enables the cutting tool to penetrate into the workpiece until
the desired depth of cut is achieved

3. A feed motion that repeats the cutting action every round or every stroke
to ensure continuation of the cutting operation

361

362 10 Machining of Metals

10.1 TURNING OPERATIONS

The Lathe and Its Construction

A lathe is a machine tool used for producing surfaces of revolution and flat edges.
Based on their purpose, construction, number of tools that can simultaneously be
mounted, and degree of automation, lathes, or more accurately, lathe-type machine
tools, can be classified as follows:

1. Engine lathes

2. Toolroom lathes

3. Turret lathes

4. Vertical turning and boring mills

5. Automatic lathes

6. Special-purpose lathes

In spite of the diversity of lathe-type machine tools, there are common features with
respect to construction and principles of operation. These features can be illustrated by
considering the commonly used representative type, the engine lathe, which is shown
in Figure 10.1. Following is a description of each of the main elements of an engine
lathe.

Lathe bed. The lathe bed is the main frame, a horizontal beam on two vertical sup-
ports. It is usually made of gray or nodular cast iron to damp vibrations and is made
by casting. It has guideways that allow the carriage to slide easily lengthwise. The
height of the lathe bed should be such that the technician can do his or her job easily
and comfortably.

Headstock. The headstock assembly is fixed at the left-hand side of the lathe bed and
includes the spindle, whose axis is parallel to the guideways (the slide surface of the
bed). The spindle is driven through the gearbox, which is housed within the headstock.
The function of the gearbox is to provide a number of different spindle speeds (usually
6 to 18 speeds). Some modern lathes have headstocks with infinitely variable spindle
speeds and that employ frictional, electrical, or hydraulic drives.

The spindle is always hollow (i.e., it has a through hole extending lengthwise).
Bar stocks can be fed through the hole if continuous production is adopted. Also, the
hole has a tapered surface to allow the mounting of a plain lathe center, such as the one
shown in Figure 10.2. It is made of hardened tool steel. The part of the lathe center that
fits into the spindle hole has a Morse taper, while the other part of the center is coni-
cal with a 60° apex angle. As explained later, lathe centers are used for mounting long
workpieces. The outer surface of the spindle is threaded to allow the mounting of a
chuck, a faceplate, or the like.

Tailstock. The tailstock assembly consists basically of three parts: its lower base, an
intermediate part, and the quill. The lower base is a casting that can slide on the lathe

10.1 Turning Operations

363

FIGURE 10.1

An engine lathe
(Courtesy of Clausing
Industrial, Inc.,
Kalamazoo, Michigan)

Headstock assembly

Tool post
Compound rest

Center
Tailstock quill

Tailstock assembly
Bed
Lead screw
Feed rod

bed along the guideways, and it has a clamping device so that the entire tailstock can
be locked at any desired location, depending upon the length of the workpiece. The in-
termediate part is a casting that can be moved transversely so that the axis of the tail-
stock can be aligned with that of the headstock. The third part, called the quill, is a
hardened steel tube that can be moved longitudinally in and out of the intermediate
part as required. This is achieved through the use of a handwheel and a screw, around
which a nut fixed to the quill is engaged. The hole in the open side of the quill is ta-
pered to allow the mounting of lathe centers or other tools like twist drills or boring
bars. The quill can be locked at any point along its travel path by means of a clamping
device.

Carriage. The main function of the carriage is to mount the cutting tools and gener-
ate longitudinal and /or cross feeds. It is actually an H-shaped block that slides on the
lathe bed between the headstock and tailstock while being guided by the V-shaped

FIGURE 10.2

A plain lathe center

Taper

364 10 Machining of Metals

guideways of the bed. The carriage can be moved either manually or mechanically by
means of the apron and either the feed rod or the lead screw.

The apron is attached to the saddle of the carriage and serves to convert the rotary
motion of the feed rod (or lead screw) into linear longitudinal motion of the carriage
and, accordingly, the cutting tool (i.e., it generates the axial feed). The apron also pro-
vides powered motion for the cross slide located on the carriage. Usually, the tool post
is mounted on the compound rest, which is, in turn, mounted on the cross slide. The
compound rest is pivoted around a vertical axis so that the tools can be set at any de-
sired angle with respect to the axis of the lathe (and that of the workpiece). These var-
ious components of the carriage form a system that provides motion for the cutting
tool in two perpendicular directions during turning operations.

When cutting screw threads, power is provided from the gearbox to the apron by
the lead screw. In all other turning operations, it is the feed rod that drives the carriage.
The lead screw goes through a pair of half nuts that are fixed to the rear of the apron.
When actuating a certain lever, the half nuts are clamped together and engage with the
rotating lead screw as a single nut that is fed, together with the carriage, along the bed.
When the lever is disengaged, the half nuts are released and the carriage stops. On the
other hand, when the feed rod is used, it supplies power to the apron through a worm
gear. This gear is keyed to the feed rod and travels with the apron along the feed rod,
which has a keyway extending along its entire length. A modern lathe usually has a
quick-change gearbox located under the headstock and driven from the spindle through
a train of gears. It is connected to both the feed rod and the lead screw so that a vari-
ety of feeds can easily and rapidly be selected by simply shifting the appropriate
levers. The quick-change gearbox is employed in plain turning, facing, and thread-
cutting operations. Because the gearbox is linked to the spindle, the distance that the
apron (and the cutting tool) travels for each revolution of the spindle can be controlled
and is referred to as the feed.

The Turret Lathe

A turret lathe is similar to an engine lathe, except that the conventional tool post is re-
placed with a hexagonal (or octagonal) turret that can be rotated around a vertical axis
as required. Appropriate tools are mounted on the six (or eight) sides of the turret. The
length of each tool is adjusted so that, by simply indexing the turret, any tool can be
brought into the exactly desired operating position. These cutting tools can, therefore,
be employed successively without the need for dismounting the tool and mounting a
new one each time, as is the case with conventional engine lathes. This results in an
appreciable saving in the time required for setting up the tools. Also, on a turret lathe,
a skilled machinist is required only initially to set up the tools. A laborer with limited
training can operate the turret lathe thereafter and produce parts identical to those that
can be manufactured when a skilled machinist operates the lathe. Figure 10.3 illus-
trates a top view of a hexagonal turret with six different tools mounted on its sides.
Sometimes, the turret replaces the tailstock and can be either vertical (i.e., with a hor-
izontal axis) or horizontal (i.e., with a vertical axis). In this case, four additional tools
can be mounted on the square tool post, sometimes called a square turret, thus allow-

10.1 Turning Operations

365

FIGURE 10.3

Top view of a hexagonal
turret with six different
tools

Reamer

ing twelve machining operations to be performed successively. Turret lathes always
have work-holding devices with quick-release (and quick-tightening) mechanisms.

Specifying a Lathe

It is important for a manufacturing engineer to be able to specify a lathe in order to
place an order or to compare and examine contract bids. The specifications of a lathe
should involve data that reveal the dimensions of the largest workpiece to be machined
on that lathe. They also must include the power consumption, as well as information
that is needed for shipping and handling. Table 10.1 indicates an example of how to
specify a lathe.

Tool Holding

Tools for turning operations are mounted in a toolholder (tool post). On an engine
lathe, it is located on the compound rest. More than one cutting tool (up to four) can
be mounted in the toolholder in order to save the time required for changing and set-
ting up each tool should only one tool be mounted at a time. In all turning operations,
the following conditions for holding the tools must be fulfilled:

1. The tip of the cutting edge must fully coincide with the level of the lathe axis.
This can be achieved by using the pointed edge of the lathe center as a basis for
adjustment, as shown in Figure 10.4. Failure to meet this condition results in a
change in the values of the cutting angles from the desired ones.

2. The centerline of the cutting tool must be horizontal.

3. The tool must be fixed tightly along its length and not just on two points.

4. A long tool-overhang should be avoided in order to eliminate any possibility for
elastic strains and vibrations.

366

10 Machining of Metals

TABLE 10.1

Example of
specifications of a lathe

/ Model

Example \

Maximum swing over bed (largest diameter

12 in. (300 mm)

of workpiece)

Maximum swing over carriage (largest

8 in. (200 mm)

diameter over carriage)

Hole through spindle

0.75 in. (19 mm)

Height of centers

6 in. (150 mm)

Turning length

24 in. (600 mm)

Thread on spindle nose

Taper in spindle and tailstock sleeves

3 Morse

21 spindle speeds

20-2000 rev/min.

Metric threads

2-6 mm

Whitworth

4-28 teeth

Feeds per revolution

0.0002-0.008 in. (0.05-0.2 mm)

Power required

1.6 kW

Net weight

1 ton

Floor space requirement

64/36/56 in.

(length/width/height)

(1600/900/1400 mm)

Lathe Cutting Tools

The shape and geometry of lathe cutting tools depend upon the purpose for which they
are employed. Turning tools can be classified into two main groups: external cutting
tools and internal cutting tools.

Types of tools. Each of these groups includes the following types of tools:

1. Turning tools. Turning tools can be either finishing or rough turning tools. Rough
turning tools have small nose radii and are employed when deep cuts are made.
Finishing tools have larger nose radii and are used when shallower cuts are made
in order to obtain the final required dimensions with good surface finish. Rough
turning tools can be right-hand or left-hand tools, depending upon the direction of
feed. They can have straight, bent, or offset shanks. Figure 10.5 illustrates the dif-
ferent kinds of turning tools.

FIGURE 10.4

A simple method for
tool setup

Workpiece

10.1 Turning Operations

367

FIGURE 10.5

Different kinds of
turning tools

Right-hand Left-hand

Rough turning tools

Broad-nose

Finishing tools

2. Facing tools. Facing tools are employed in facing operations for machining fiat
side or end surfaces. As can be seen in Figure 10.6, there are tools for machining
both left and right side surfaces. These side surfaces are generated through the use
of cross feed, contrary to turning operations, where longitudinal feed is used.

3. Cutoff tools. Cutoff tools, which are sometimes called parting tools, serve to sepa-
rate the workpiece into parts and/or machine external annular grooves, as shown in
Figure 10.7.

4. Thread-cutting tools. Thread-cutting tools have either triangular, square, or trape-
zoidal cutting edges, depending upon the cross section of the desired thread. Also,
the plane angles of these tools must always be identical to those of the thread
forms. Thread-cutting tools have straight shanks for external thread cutting and
bent shanks for internal thread cutting. Figure 10.8 illustrates the different shapes
of thread-cutting tools.

FIGURE 10.6

Different kinds of
facing tools

FIGURE 10.7

Cutoff tools

J-

368

10 Machining of Metals

FIGURE 10.8

Different shapes of
thread-cutting tools

Triangular

Square

Trapezoidal

5. Form tools. As shown in Figure 10.9, form tools have edges specially manufactured
to take a form that is opposite to the desired shape of the machined workpiece.

Internal and external tools. The types of internal cutting tools are similar to those of
the external cutting tools. They include tools for rough turning, finish turning, thread
cutting, and recess machining. Figure 10.10 illustrates the different types of internal
cutting tools.

Carbide tips. As previously mentioned, a high-speed steel tool is usually made in
the form of a single piece, contrary to cemented carbides or ceramics, which are
made in the form of tips. The tips are brazed or mechanically fastened to steel
shanks. Figure 10.11 shows an arrangement that includes a carbide tip, a chip
breaker, a seat, a clamping screw (with a washer and a nut), and a shank. As its name
suggests, the function of a chip breaker is to break long chips every now and then,
thus preventing the formation of very long, twisted ribbons that may cause problems
during the machining operation. As shown in Figure 10.12, the carbide tips (or ce-
ramic tips) have different shapes, depending upon the machining operations for
which they are to be employed. The tips can either be solid or have a central through
hole, depending upon whether brazing or mechanical clamping is employed for
mounting the tip on the shank.

FIGURE 10.9

Form tools

O"
^

/"~\

K-S

FIGURE 10.10

Different types of
internal cutting tools

YMM

Recess or
groove making

nm\ u

Internal
threading

10.1 Turning Operations

369

FIGURE 10.11

A carbide tip fastened
to a toolholder

Clamp

FIGURE 10.12

Different shapes of
carbide tips

Round

Diamond

Square

Triangle

Methods of Supporting Workpieces
in Lathe Operations

Some precautions must be taken when mounting workpieces on a lathe to ensure
trouble-free machining. They can be summarized as follows:

1. It is recommended that an appropriate gripping force that is neither too high nor too
low be used. A high gripping force may result in distortion of the workpiece after
the turning operation, whereas a low gripping force causes either vibration of the
workpiece or slip between the workpiece and the spindle (i.e., the rotational speed,
or rpm, of the workpiece will be lower than that of the spindle).

2. The workpiece must be fully balanced, both statically and dynamically, by em-
ploying counterweights and the like if necessary.

3. The cutting force should not affect the shape of the workpiece or cause any perma-
nent deformation. A manufacturing engineer should calculate the cutting force using
his or her knowledge of metal cutting (Chapter 9) and then check whether or not such
a force will cause permanent deformation by using stress analysis. Such calculations
are very important when machining slender workpieces (i.e., those with high length-
to-diameter ratios). Whenever it becomes evident that the cutting force will cause
permanent deformation, the machining parameters must be changed to reduce the
magnitude of the force (e.g., use a smaller depth of cut or lower feed).

Following is a brief discussion of each of the work-holding methods employed in
lathe operations.

Holding the workpiece between two centers. The workpiece is held between two
centers when turning long workpieces like shafts and axles having length-to-diameter
ratios higher than 3 or 4. Before a workpiece is held, each of its flat ends must be pre-
pared by drilling a 60° center hole. The pointed edges of the live center (mounted in
the tailstock so that its conical part rotates freely with the workpiece) and the dead
center (mounted in the spindle hole) are inserted in the previously drilled center holes.

370

10 Machining of Metals

FIGURE 10.13

Holding the workpiece
between two centers
during turning

Faceplate

(screwed on the

spindle nose)

As shown in Figure 10.13, a driving dog is clamped on the left end of the workpiece
by means of a tightening screw. The tail of the lathe dog enters a slot in the driving-
dog plate (or faceplate), which is screwed on the spindle nose.

When very long workpieces having length-to-diameter ratios of 10 or more are
turned between centers, rests must be used to provide support and prevent sagging of
the workpiece at its middle. Steady rests are clamped on the lathe bed and thus do not
move during the machining operation; follower rests are bolted to and travel with the
carriage. A steady rest employs three adjustable fingers to support the workpiece.
However, in high-speed turning, the steady rest should involve balls and rollers at the
end of the fingers where the workpiece is supported. A follower rest has only two fin-
gers and supports the workpiece against the cutting tool. A steady rest can be used as
an alternative to the tailstock for supporting the right-hand end of the workpiece. Fig-
ure 10.14 illustrates a steady rest used to support a very long workpiece.

Holding the workpiece in a chuck. When turning short workpieces and/or when per-
forming facing operations, the workpiece is held in a chuck, which is screwed on the
spindle nose. A universal, self-centering chuck has three jaws that can be moved sep-

FIGURE 10.14

A steady rest used to
support a very long
workpiece

Three adjustable
aws

10.1 Turning Operations

371

arately or simultaneously in radial slots toward its center to grip the workpiece or away
from its center to release the workpiece. This movement is achieved by inserting a
chuck wrench into a square socket and then turning it as required. Four-jaw chucks are
also employed; these are popular when turning complex workpieces and those having
asymmetric shapes. Magnetic chucks (without jaws) are used to hold thin, fiat work-
pieces for facing operations. There are also pneumatic and hydraulic chucks, and they
are utilized for speeding up the processes of loading and unloading the workpieces.
Figure 10.15 shows how a workpiece is held in a chuck.

Mounting the workpiece on a faceplate. A faceplate is a large circular disk with ra-
dial plain slots and T-slots in its face. The workpiece can be mounted on it with the
help of bolts, T-nuts, and other means of clamping. The faceplate is usually employed
when the workpiece to be gripped is large or noncircular or has an irregular shape and
cannot, therefore, be held in a chuck. Before any machining operation, the faceplate
and the workpiece must be balanced by a counterweight mounted opposite to the
workpiece on the faceplate, as shown in Figure 10.16.

Using a mandrel. Disklike workpieces or those that have to be machined on both
ends are mounted on mandrels, which are held between the lathe centers. In this case,
the mandrel acts like a fixture and can take different forms. As Figure 10.17 shows, a

FIGURE 10.15

Holding the workpiece
in a chuck

Chuck
body

Chuck
jaw

Large bevel gear
with spiral scrol
on the other side

FIGURE 10.16

Mounting the workpiece
on a faceplate

Faceplate

Counterweight

Workpiece

372

10 Machining of Metals

FIGURE 10.17

Mounting the workpiece
on a mandrel

Flattened

surface

(for the dog)

Workpiece

y; // ;; ;/ // // />

Lathe
center

Mandrel having
invisible slope

Tightening
nut

mandrel can be a truncated conical rod with an intangible slope on which the work-
piece is held by the wedge action. A split sleeve that is forced against a conical rod is
also employed. There are also some other designs for mandrels.

Holding the workpiece in a chuck collet. A chuck collet consists of a three-segment
split sleeve with an external tapered surface. The collet can grip a smooth bar placed
between these segments when a collet sleeve, which is internally tapered, is pushed
against the external tapered surface of the split sleeve, as shown in Figure 10.18.

Lathe Operations

The following sections focus on the various machining operations that can be per-
formed on a conventional engine lathe. It must be born in mind, however, that modern
computerized numerically controlled (CNC) lathes have more capabilities and can do
other operations, such as contouring, for example. Following are the conventional
lathe operations.

Cylindrical turning. Cylindrical turning is the simplest and the most common of all
lathe operations. A single full turn of the workpiece generates a circle whose center
falls on the lathe axis; this motion is then reproduced numerous times as a result of the
axial feed motion of the tool. The resulting machining marks are, therefore, a helix
having a very small pitch, which is equal to the feed. Consequently, the machined sur-
face is always cylindrical.

The axial feed is provided by the carriage or compound rest, either manually or
automatically, whereas the depth of cut is controlled by the cross slide. In roughing
cuts, it is recommended that large depths of cuts, up to 1/4 inch (6 mm) depending
upon the workpiece material, and smaller feeds be used. On the other hand, very fine
feeds, smaller depths of cut, less than 0.05 inch (0.4 mm), and high cutting speeds are

FIGURE 10.18

Holding the workpiece
in a chuck collet

Workpiece

10.1 Turning Operations

373

FIGURE 10.19

Equations applicable to lathe operations

Operation

Cutting Speed

Machining Time

Material -removal Rate

N (rpm)

Turning
(external)

V = it(D +2d)N

fN
where L = *.workpieCe + allowance
i.e., length of the workpiece plus

allowance

MRR = tt(0 + d)N-f-d

f (feed)

Boring

V = nDN

T-k

MRR = n(D - d)N-f-d

Facing

Feed, f

max. V = nDN
min. V = 0

.. nDN

mean y = — - —

D + allowance
2fN

max. MRR = nDN-f-d
mean MRR =

Parting

max. V = nDN
min. V = 0

■nDN
2

D + allowance
2fN

mean 1/

max. MRR = nDN-f-d

MDD irDN-f-d
mean MRR =

Feed, f

preferred for finishing cuts. Figure 10.19 indicates the equations used to estimate the
different machining parameters in cylindrical turning.

Facing. The result of a facing operation is a flat surface that is either the entire end
surface of the workpiece or an annular intermediate surface like a shoulder. During a
facing operation, feed is provided by the cross slide, whereas the depth of cut is con-
trolled by the carriage or compound rest. Facing can be carried out either from the pe-
riphery inward or from the center of the workpiece outward. It is obvious that the
machining marks in both cases take the form of a spiral. Usually, it is preferred to
clamp the carriage during a facing operation as the cutting force tends to push the tool
(and, of course, the whole carriage) away from the workpiece. In most facing opera-
tions, the workpiece is held in a chuck or on a faceplate. Figure 10.19 also indicates
the equations applicable to facing operations.

374

10 Machining of Metals

Groove cutting. In cutoff and groove-cutting operations, only cross feed of the tool is
employed. The cutoff and grooving tools that were previously discussed are employed.

Boring and internal turning. Boring and internal turning are performed on the inter-
nal surfaces by a boring bar or suitable internal cutting tool. If the initial workpiece is
solid, a drilling operation must be performed first. The drilling tool is held in the tail-
stock, which is then fed against the workpiece.

Taper turning. Taper turning is achieved by driving the tool in a direction that is not
parallel to the lathe axis but inclined to it with an angle that is equal to the desired
angle of the taper. Following are the different methods used in taper turning:

1. One method is to rotate the disk of the compound rest with an angle equal to half the
apex angle of the cone, as is shown in Figure 10.20. Feed is manually provided by
cranking the handle of the compound rest. This method is recommended for the taper
turning of external and internal surfaces when the taper angle is relatively large.

2. Special form tools can be used for external, very short, conical surfaces, as shown
in Figure 10.21. The width of the workpiece must be slightly smaller than that of
the tool, and the workpiece is usually held in a chuck or clamped on a faceplate. In

FIGURE 10.20

Taper turning by
rotating the disk of the
compound rest

Workpiece

Chuck

FIGURE 10.21

Taper turning by
employing a form tool

Workpiece

Tool

Chuck

10.1 Turning Operations

375

this case, only the cross feed is used during the machining process, and the carriage
is clamped to the machine bed.

The method of offsetting the tailstock center, as shown in Figure 10.22, is em-
ployed for the external taper turning of long workpieces that are required to have
small taper angles (less than 8°). The workpiece is mounted between the two cen-
ters; then the tailstock center is shifted a distance S in the direction normal to the
lathe axis. This distance can be obtained from the following equation:

5 =

L(D - d)

(10.11

where: L is the full length of the workpiece

D is the largest diameter of the workpiece
d is the smallest diameter of the workpiece
i is the length of the tapered surface

4. A special taper-turning attachment, such as the one shown in Figure 10.23, is used
for turning very long workpieces, when the length is larger than the full stroke of
the compound rest. The procedure followed in such cases involves complete disen-
gagement of the cross slide from the carriage, which is then guided by the taper-
turning attachment. During this process, the automatic axial feed can be used as
usual. This method is recommended for very long workpieces with a small cone
angle (8° through 10°).

Thread cutting. For thread cutting, the axial feed must be kept at a constant rate,
which is dependent upon the rotational speed (rpm) of the workpiece. The relationship
between both is determined primarily by the desired pitch of the thread to be cut.

As previously mentioned, the axial feed is automatically generated when cutting a
thread by means of the lead screw, which drives the carriage. When the lead screw rotates

FIGURE 10.22

Taper turning by
offsetting the tailstock
center

376

10 Machining of Metals

FIGURE 10.23

Taper turning by
employing a special
attachment

a single revolution, the carriage travels a distance equal to the pitch of the lead screw. Con-
sequently, if the rotational speed of the lead screw is equal to that of the spindle (i.e., that
of the workpiece), the pitch of the resulting cut thread is exactly equal to that of the lead
screw. The pitch of the resulting thread being cut, therefore, always depends upon the ratio
of the rotational speeds of the lead screw and the spindle:

pitch of lead screw

rpm of workpiece

desired pitch of workpiece rpm of lead screw

= spindle-to-carriage gearing ratio

(10.2)

This equation is useful in determining the kinematic linkage between the lathe spindle
and the lead screw and enables proper selection of the gear train between them.

In thread-cutting operations, the workpiece can be either held in a chuck or
mounted between two lathe centers for relatively long workpieces. The form of the
tool used must exactly coincide with the profile of the thread to be cut (i.e., triangular
tools must be used for triangular threads, and so on).

Knurling. Knurling is basically a forming operation in which no chips are produced.
It involves pressing two hardened rolls with rough filelike surfaces against the rotat-
ing workpiece to cause plastic deformation of the workpiece metal, as shown in Fig-
ure 10.24. Knurling is carried out to produce rough, cylindrical (or conical) surfaces
that are usually used as handles. Sometimes, surfaces are knurled just for the sake of
decoration, in which case there are different knurl patterns to choose from.

Cutting Speeds and Feeds

The cutting speed, which is usually given in surface feet per minute (SFM), is the
number of feet traveled in the circumferential direction by a given point on the surface
(being cut) of the workpiece in one minute. The relationship between the surface speed

10.1 Turning Operations

377

FIGURE 10.24

The knurling operation

and the rpm can be given by the following equation:
SFM = kDN (see Table 1 0. 1 )
where: D is the diameter of the workpiece in feet
TV is the rpm

The surface cutting speed is dependent upon the material being machined as well
as the material of the cutting tool and can be obtained from handbooks and informa-
tion provided by cutting-tool manufacturers. Generally, the SFM is taken as 100 when
machining cold-rolled or mild steel, as 50 when machining tougher metals, and as 200
when machining softer materials. For aluminum, the SFM is usually taken as 400 or
above. There are also other variables that affect the optimal value of the surface cut-
ting speed. These include the tool geometry, the type of lubricant or coolant, the feed,
and the depth of cut. As soon as the cutting speed is decided upon, the rotational speed
(rpm) of the spindle can be obtained as follows:

/V =

SFM
kD

(10.3)

The selection of a suitable feed depends upon many factors, such as the required
surface finish, the depth of cut, and the geometry of the tool used. Finer feeds will pro-
duce better surface finish, whereas higher feeds reduce the machining time during which
the tool is in direct contact with the workpiece. Therefore, it is generally recommended
to use high feeds for roughing operations and finer feeds for finishing operations. Again,
recommended values for feeds, which can be taken as guidelines, are found in hand-
books and in information booklets provided by cutting-tool manufacturers.

Design Considerations for Turning

When designing parts to be produced by turning, the product designer must consider
the possibilities and limitations of the turning operation as well as the machining cost.
The cost increases with the quality of the surface finish, with the tightness of the tol-
erances, and with the area of the surface to be machined. Therefore, it is not recom-
mended that high-quality surface finishes or tighter tolerances be used in the product
design unless they are required for the proper functioning of the product. Figure 10.25

378

10 Machining of Metals

FIGURE 10.25

Design considerations
for turning: (a) reduce
area of surface to be
machined; (b) reduce
number of operations
required; (c) provide
allowance for tool
clearance; (d) opt for
machining external over
internal surfaces;
(e) opt for through
boring over alternatives

R^v^^:

>*■ LJ

(a)

Not recommended Preferred

(b)

Preferred Not recommended

(d)

Cover

Better design

Less recommended

(e)

graphically depicts some design considerations for turning. Here are the guidelines to
be followed:

1. Try to reduce the area of the surfaces to be machined, especially when a large num-
ber of parts is required or when the surfaces are to mate with other parts (see Fig-
ure 10.25a).

2. Try to reduce the number of operations required by appropriate changes in the de-
sign (see Figure 10.25b).

3. Provide an allowance for tool clearance between different sections of a product (see
Figure 10.25c).

4. Always keep in mind that machining of exposed surfaces is easier and less expen-
sive than machining of internal surfaces (see Figure 10.25d).

10.2 Shaping and Planing Operations 379

5. Remember that through boring is easier and cheaper than other alternatives (see
Figure 10.25e).

SHAPING AND PLANING OPERATIONS

Planing, shaping, and slotting are processes for machining horizontal, vertical, and in-
clined flat surfaces, slots, or grooves by means of a lathe-type cutting tool. In all these
processes, the cutting action takes place along a straight line. In planing, the workpiece
(and the machine bed) is reciprocated, and the tool is fed across the workpiece to re-
produce another straight line, thus generating a flat surface. In shaping and slotting, the
cutting tool is reciprocated, and the workpiece is fed normal to the direction of tool
motion. The difference between the latter two processes is that the tool path is hori-
zontal in shaping and it is vertical in slotting. Shapers and slotters can be employed in
cutting external and internal keyways, gear racks, dovetails, and T-slots. Shapers and
planers have become virtually obsolete because most shaping and planing operations
have been replaced by more productive processes such as milling, broaching, and abra-
sive machining. The use of shapers and planers is now limited to the machining of
large beds of machine tools and the like.

In all three processes, there are successive alternating cutting and idle return
strokes. The cutting speed is, therefore, the speed of the tool (or the workpiece) in the
direction of cutting during the working stroke. The cutting speed may be either con-
stant throughout the working stroke or variable, depending upon the design of the
shaper or planer. Let us now discuss the construction and operation of the most com-
mon types of shapers and planers.

Horizontal Push-Cut Shaper

Construction. As can be seen in Figure 10.26, a horizontal push-cut shaper consists
of a frame that houses the speed gearbox and the quick-return mechanism that trans-
mits power from the motor to the ram and the table. The ram travel is the primary mo-
tion that produces a straight-line cut in the working stroke, whereas the intermittent
cross travel of the table is responsible for the cross feed. The tool head is mounted at
the front end of the ram and carries the clapper box toolholder. The toolholder is piv-
oted at its upper end to allow the tool to rise during the idle return stroke in order not
to ruin the newly machined surface. The tool head can be swiveled to permit the ma-
chining of inclined surfaces.

The workpiece can be either bolted directly to the machine table or held in a vise
or other suitable fixture. The cross feed of the table is generated by a ratchet and pawl
mechanism that is driven through the quick-return mechanism (i.e., the crank and the
slotted arm). The machine table can be raised or lowered by means of a power screw
and a crank handle. It can also be swiveled in a universal shaper.

Quick-return mechanism. As can be seen in Figure 10.27, the quick-return mecha-
nism involves a rotating crank that is driven at a uniform angular speed and an oscil-
lating slotted arm that is connected to the crank by a sliding block. The working stroke
takes up an angle (of the crank revolution) that is larger than that of the return stroke.

380

10 Machining of Metals

FIGURE 10.26

Design features of a
horizontal push-cut
shaper

Feed screw

(to control

depth of cut)

Tool slide

Clapper
box

■Table

Screw
for adjusting
table height

Because the angular speed of the crank is constant, it is obvious that the time taken by
the idle return stroke is less than that taken by the cutting stroke. In fact, it is the main
function of the quick-return mechanism to reduce the idle time during the machining
operation to a minimum.

Now, let us consider the average speed (s) of the tool during the cutting stroke. It
can be determined as a function of the length of the stroke and the number of strokes
per minute as follows:

2LN . . , . , . . ,

5 = in tt/min. (m/min.)

where: L is the length of stroke in feet (m)

TV is the number of strokes per minute
C is the cutting time ratio

Note that the cutting time ratio is

cutting time

(10.4)

C =

total time for one crank revolution

_ angle corresponding to cutting stroke

It is also obvious that the total number of strokes required to machine a given surface
can be given by the following equation:

W_
f

(10.5)

10.2 Shaping and Planing Operations

381

FIGURE 10.27

Details and working
principles of the quick-
return mechanism

Length of stroke

Sliding
block

where: W is the total width of the workpiece

/is the cross feed (e.g., inches per stroke)
Therefore, the machining time T is n/N. After mathematical manipulation, it can be
given as follows:

2LN (10.6)

T =

sxC

382 10 Machining of Metals

Next, the metal-removal rate (MRR) can be given by the following equation:

MRR = rx/xLxyV(in.3/min.) (10.7)

Vertical Shaper

The vertical shaper is similar in construction and operation to the push-cut shaper, the
difference being that the ram and the tool head travel vertically instead of horizontally.
Also, in this type of shaper, the workpiece is mounted on a round table that can have
a rotary feed whenever desired to allow the machining of curved surfaces (e.g., spiral
grooves). Vertical shapers, which are sometimes referred to as slotters, are used in in-
ternal cutting. Another type of vertical shaper is known as a keyseater because it is
specially designed for cutting keyways in gears, cams, pulleys, and the like.

Planer

A planer is a machine tool that does the same work as the horizontal shaper but on work-
pieces that are much larger than those machined on a shaper. Although the designs of
planers vary, most common are the double-housing and open-side constructions. In a
double-housing planer, two vertical housings are mounted at the sides of the long, heavy
bed. A cross rail that is supported at the top of these housings carries the cutting tools.
The machine table (while in operation) reciprocates along the guide ways of the bed and
has T-slots in its upper surface for clamping the workpiece. In this type of planer, the
table is powered by a variable-speed dc motor through a gear drive. The cross rail can
be raised or lowered as required, and the inclination of the tools can be adjusted as well.
In an open-side planer, there is only one upright housing at one side of the bed. This con-
struction provides more flexibility when wider workpieces are to be machined.

Planing and Shaping Tools

Planing and shaping processes employ single-point tools of the lathe type, but heavier
in construction. They are made of either high-speed steel or carbon tool steel with car-
bide tips. In the latter case, the machine tool should be equipped with an automatic lift-
ing device to keep the tool from rubbing the workpiece during the return stroke, thus
eliminating the possibility of breaking or chipping the carbide tips.

The cutting angles for these tools depend upon the purpose for which the tool is
to be used and the material being cut. The end relief angle does not usually exceed 4°,
whereas the side relief varies between 6° and 14°. The side rake angle also varies be-
tween 5° (for cast iron) and 15° (for medium-carbon steel).

DRILLING OPERATIONS

Drilling involves producing through or blind holes in a workpiece by forcing a tool
that rotates around its axis against the workpiece. Consequently, the range of cutting
from this axis of rotation is equal to the radius of the required hole. In practice, two
symmetrical cutting edges that rotate about the same axis are employed.

10.3 Drilling Operations

383

Drilling operations can be carried out by using either hand drills or drilling ma-
chines. The latter differ in size and construction. Nevertheless, the tool always rotates
around its axis while the workpiece is kept firmly fixed. This is contrary to drilling on
a lathe.

Cutting Tools for Drilling Operations

In drilling operations, a cylindrical rotary-end cutting tool, called a drill, is employed.
The drill can have one or more cutting edges and corresponding flutes that are straight
or helical. The function of the flutes is to provide outlet passages for the chips gener-
ated during the drilling operation and also to allow lubricants and coolants to reach the
cutting edges and the surface being machined. Following is a survey of the commonly
used types of drills.

Twist drill. The twist drill is the most common type of drill. It has two cutting edges
and two helical flutes that continue over the length of the drill body, as shown in Fig-
ure 10.28. The drill also consists of a neck and a shank that can be either straight or ta-
pered. A tapered shank is fitted by the wedge action into the tapered socket of the
spindle and has a tang that goes into a slot in the spindle socket, thus acting as a solid
means for transmitting rotation. Straight-shank drills are held in a drill chuck that is,
in turn, fitted into the spindle socket in the same way as tapered-shank drills.

As can be seen in Figure 10.28, the two cutting edges are referred to as the lips
and are connected together by a wedge, which is a chisel-like edge. The twist drill also
has two margins that allow the drill to be properly located and guided while it is in op-
eration. The tool point angle (TPA) is formed by the two lips and is chosen based on
the properties of the material to be cut. The usual TPA for commercial drills is 118°,
which is appropriate for drilling low-carbon steels and cast irons. For harder and
tougher metals, such as hardened steel, brass, and bronze, larger TPAs (130° or 140°)

FIGURE 10.28

A twist drill

Body

Tang

Tool point
angle

Chisel

edge

(wedge

Margin

Flute

384

10 Machining of Metals

give better performance. The helix angle of the flutes of a twist drill ranges between
24° and 30°. When drilling copper or soft plastics, higher values for the helix angle are
recommended (between 35° and 45°). Twist drills are usually made of high-speed
steel, although carbide-tipped drills are also available. The sizes of twist drills used in
industrial practice range from 0.01 inch to V/i inches (0.25 up to 80 mm).

Core drill. A core drill consists of the chamfer, body, neck, and shank, as shown in
Figure 10.29. This type of drill may have three or four flutes and an equal number of
margins, which ensures superior guidance, thus resulting in high machining accuracy.
The figure also shows that a core drill has a flat end. The chamfer can have three or
four cutting edges, or lips, and the lip angle may vary between 90° and 120°. Core
drills are employed for enlarging previously made holes and not for originating holes.
This type of drill promotes greater productivity, high machining accuracy, and superior
quality of the drilled surfaces.

Gun drill. A gun drill is used for drilling deep holes. All gun drills are straight-fluted,
and each has a single cutting edge. A hole in the body acts as a conduit to trans-
mit coolant under considerable pressure to the tip of the drill. As can be seen in Fig-
ure 10.30, there are two kinds of gun drills: the center-cut gun drill used for drilling
blind holes and the trepanning drill. The latter has a cylindrical groove at its center,
thus generating a solid core that guides the tool as it proceeds during the drilling op-
eration.

Spade drill. A spade drill is used for drilling large holes of 3V2 inches (90 mm) or
more. The design of this type of drill results in a marked saving in tool cost as well as
in a tangible reduction in tool weight that facilitates its ease of handling. Moreover,
this drill is easy to grind. Figure 10.31 shows a spade drill.

Saw-type cutter. A saw-type cutter, like the one illustrated in Figure 10.32, is used
for cutting large holes in thin metal.

Drills made in combination with other tools. An example is a tool that involves both
a drill and a tap. Step drills and drill and countersink tools are also sometimes used in
industrial practice.

Cutting Speeds and Feeds in Drilling

We can easily see that the cutting speed varies along the cutting edge. It is always
maximum at the periphery of the tool and is equal to zero on the tool axis. Never-
theless, we consider the maximum speed because it is the one that affects the tool

FIGURE 10.29

A core drill

Chamfer

TPA

n _■ Neck

Body 1 i Shank

Helix
angle

10.3 Drilling Operations

385

FIGURE 10.30

Gun drills: (a)
trepanning gun drill; (b)
center-cut gun drill

Cutting Cutting fluid
edge passage

Shape of the
resulting hole

(a)

(b)

FIGURE 10.31

A spade drill

Fastening
screw

Diameter

of the

resulting hole

FIGURE 10.32

A saw-type cutter

386

10 Machining of Metals

wear and the quality of the machined surface. The maximum speed must not exceed
the permissible cutting speed, which depends upon the material of the workpiece as
well as the material of the cutting tool. Data about permissible cutting speeds in
drilling operations can be found in handbooks. The rotational speed of the spindle
can be determined from the following equation:

N =

(10.8)

where: N is the rotational speed of the spindle (rpm)
D is the diameter of the drill in feet (m)
CS is the permissible cutting speed in ft/min. (m/min.)

In drilling operations, feeds are expressed in inches or millimeters per revolution.
Again, the appropriate value of feed to be used depends upon the metal of the work-
piece and drill material and can be found in handbooks. Whenever the production rate
must be increased, it is advisable to use higher feeds rather than increase the cutting
speed.

Other Types of Drilling Operations

In addition to conventional drilling, there are other operations that are involved in the
production of holes in industrial practice. Following is a brief description of each of
these operations.

Boring. Boring involves enlarging a hole that has already been drilled. It is similar to
internal turning and can, therefore, be performed on a lathe, as previously mentioned.
There are also some specialized machine tools for carrying out boring operations.
These include the vertical boring mill, the jig boring machine, and the horizontal bor-
ing machine.

Counterboring. As a result of counterboring, only one end of a drilled hole is en-
larged, as is illustrated in Figure 10.33a. This enlarged hole provides a space in which
to set a bolt head or a nut so that it will be entirely below the surface of the part.

Spot facing. Spot facing is performed to finish off a small surface area around the
opening of a hole. As can be seen in Figure 10.33b, this process involves removing a
minimal depth of cut and is usually performed on castings or forgings.

Countersinking. As shown in Figure 10.33c, countersinking is done to accommodate
the conical seat of a flathead screw so that the screw does not appear above the surface
of the part.

FIGURE 10.33

Operations related to
drilling:

(a) counterboring;

(b) spot facing;

10.3 Drilling Operations

387

FIGURE 10.34

Details of a reamer

Fluted section

Neck

Shank

Relief
angle

Rake angle

Tool angle

Cutting angles
of a tooth

Reaming. Reaming is actually a "sizing" process, by which an already drilled hole is
slightly enlarged to the desired size. As a result of a reaming operation, a hole has a very
smooth surface. The cutting tool used in this operation is known as a reamer. As shown
in Figure 10.34, a reamer has a fluted section, a neck, and a shank. The fluted section in-
cludes four zones: the chamfer, the starting taper, the sizing zone, and the back taper.
The chamfer or bevel encloses an angle that depends upon the method of reaming and
the material being cut. This is a consequence of the fact that this angle affects the mag-
nitude of the axial reaming force. The larger the chamfer angle, the larger the required
reaming force. Table 10.2 indicates some recommended values of the chamfer angle for
different reaming conditions. The starting taper is the part of the reamer that actually re-
moves chips. Figure 10.34 also shows that each tooth of that part of the reamer has a cut-
ting edge as well as rake, relief, and tool (or lip) angles. The sizing zone guides the
reamer and smooths the surface of the hole. Finally, the back taper serves to reduce fric-
tion between the reamer and the newly machined surface.

Reamers are usually made of hardened tool steel. Nevertheless, reamers that are
used in mass production are tipped with cemented carbides in order to increase the tool
life and the production rate.

Tapping. Tapping is the process of cutting internal threads. The tool used is called a
tap. As shown in Figure 10.35, it has a boltlike shape with four longitudinal flutes.
Made of hardened tool steel, taps can be used for either manual or machine cutting of

TABLE 10.2

Recommended values
of the chamfer angle of
reamers

Metal to Be Reamed

Steel

Cast Iron

Soft Metals

Manual reaming
Machining reaming

l°-3°
8°-10c

l°-3°
20°-30c

l°-3°
3°-5°

388

10 Machining of Metals

FIGURE 10.35

A tap

Chamfer

Land

Flute

Thread
length

threads. In the latter case, the spindle of the machine tool must reverse its direction of
rotation at the end of the cutting stroke so that the tap can be withdrawn without de-
stroying the newly cut thread. When tapping is carried out by hand, a set of three taps
is used for each desired threaded hole size. The three taps differ slightly in size, and
two of them are actually undersized. The first tap of the set to be used is always a tap-
per tap; it reduces the torque (and, consequently, the power) required for tapping.

Design Considerations for Drilling

Figure 10.36 graphically depicts some design considerations for drilling. Here are the
guidelines to be followed:

1. Make sure the centerline of the hole to be drilled is normal to the surface of the
part. This is to avoid bending and breaking the tool during the drilling operation. As
previously mentioned, the twist drill has a chisel edge and not a pointed edge at its
center. This, although it facilitates the process of grinding the tool, causes the tool
to shift from the desired location and makes it liable to breakage, especially if it is
not normal to the surface to be drilled. (See Figure 10.36a for examples of poor and
proper design practice for drilled holes.)

2. When tapping through holes, ensure that the tap will be in the clear when it appears
from the other side of the part (see Figure 10.36b).

3. Remember that it is impossible to tap the entire length of a blind or counterbored
hole without providing special tool allowance (see Figure 10.36c).

Classification of Drilling Machines

Drilling operations can be carried out by employing small portable machines or by
using the appropriate machine tools. These machine tools differ in shape and size, but
they have common features. For instance, they all involve one or more twist drills, each
rotating around its own axis while the workpiece is kept firmly fixed. This is contrary
to the drilling operation on a lathe, where the workpiece is held in and rotates with the
chuck. Following is a survey of the commonly used types of drilling machines.

Bench-type drilling machines. Bench-type drilling machines are general-purpose,
small machine tools that are usually placed on benches. This type of drilling machine
includes an electric motor as the source of motion, which is transmitted via pulleys and
belts to the spindle, where the tool is mounted. The feed is manually generated by low-

10.3 Drilling Operations

389

FIGURE 10.36

Design considerations
for drilling: (a) set
centerline of tool
normal to surface to be
drilled; (b) ensure tap is
clear when it appears
from other side;
(c) provide allowance
when tapping a blind
hole

To be avoided

Recommended

Acceptable

(a)

To be avoided

(b)

= ^ in. (6 mm)

ering a lever handle that is designed to lower (or raise) the spindle. The spindle rotates
freely inside a sleeve (which is actuated by the lever through a rack-and-pinion sys-
tem) but does not rotate with the spindle.

The workpiece is mounted on the machine table, although a special vise is some-
times used to hold the workpiece. The maximum height of a workpiece to be machined
is limited by the maximum gap between the spindle and the machine table.

Upright drilling machines. Depending upon the size, upright drilling machines can be
used for light, medium, and even relatively heavy jobs. A light-duty upright drilling ma-
chine is shown in Figure 10.37. It is basically similar to a bench-type machine, the main
difference being a longer cylindrical column fixed to the base. Along the column is an
additional sliding table for fixing the workpiece that can be locked in position at any de-
sired height. The power required for this type of machine is greater than that for a bench-
type drilling machine as this type is employed in performing medium-duty jobs.

390

10 Machining of Metals

FIGURE 10.37

An upright drilling
machine (Courtesy of
Clausing Industrial,
Inc., Kalamazoo,
Michigan)

Spindle
Drill chuck

Capstan
wheel

Table

Column

Base

There are also large drilling machines of the upright type. In this case, the ma-
chine has a box column and a higher power to deal with large jobs. Moreover, gear-
boxes are employed to provide different rotational spindle speeds as well as axial feed
motion, which can be preset at any desired rate.

Multispindle drilling machines. Multispindle drilling machines are sturdily con-
structed and require high power; each is capable of drilling many holes simultaneously.
The positions of the different tools (spindles) can be adjusted as desired. Also, the en-
tire head (which carries the spindles and the tools) can be tilted if necessary. This type
of drilling machine is used mainly for mass production in jobs having many holes,
such as cylinder blocks.

Gang drilling machines. When several separate heads (each with a single spindle) are
arranged on a single common table, the machine tool is then referred to as a gang
drilling machine. This type of machine tool is particularly suitable where several op-
erations are to be performed in succession.

10.3 Drilling Operations

391

Radial drills. Radial drills are particularly suitable for drilling holes in large and
heavy workpieces that are inconvenient to mount on the table of an upright drilling
machine. As shown in Figure 10.38, a radial drilling machine has a main column that
is fixed to the base. The cantileverd guide arm, which carries the drilling head spindle
and tool, can be raised or lowered along the column and clamped at any desired posi-
tion. The drilling head slides along the arm and provides rotary motion and axial feed
motion. The cantilevered guide arm can be swung, thus allowing the tool to be moved
in all directions according to a cylindrical coordinate system.

Turret drilling machines. Machine tools that belong in the turret drilling machine cat-
egory are either semiautomatic or fully automatic. A common design feature is that the
main spindle is replaced by a turret that carries several drilling, boring, reaming, and
threading tools. Consequently, several successive operations can be carried out with
only a single initial setup and without the need for setting up the workpiece again be-
tween operations.

Automatic turret drilling machines that are operated by NC or CNC systems (see
Chapter 14) are quite common. In this case, the human role is limited to the initial
setup and monitoring. This type of machine tool has advantages over the gang-type
drilling machine with respect to the space required (physical size of the machine tool)
and the number of workpiece setups.

Deep-hole drilling machines. Deep-hole drilling machines are special machines em-
ployed for drilling long holes like those of rifle barrels. Usually, gun-type drills are
used and are fed slowly against the workpiece. In this type of machine tool,